From two-for-ones to baker’s dozens, retailers have long offered discounts for bulk purchases. But the travel industry inverts such practices by sometimes charging more if you book seats for more than one passenger, requiring you to strategize before buying tickets for itineraries with several people in your traveling party.

It’s all due to the complexity of the airlines’ pricing practices. Many of us know from experience that the price of the same seat on the same flight can seem to fluctuate day by day—even hour by hour. Less well-known is the fact that the same airline seat can be had at different prices at the exact same time and that the number of seats you’re booking at once can determine the price you are shown.

In a series of online searches of different airline and travel booking sites, we found that the price difference could be considerable: up to $103 per person on an American flight from New York to Los Angeles, when we tried to book 6 seats compared with just one.

But, fortunately, if you’re planning a trip this summer with a group of friends or family, or even just one other person, there are a few simple tactics that can help you get a lower fare.

How Airlines Price Seats

Experts who study airline pricing explain the price discrepancies by noting that an average domestic flight can offer about 10 different prices for the same seat in an economy class cabin—and that’s not counting premium economy or the newer basic economy.

Let’s say a given flight contains 20 bookable economy seats, with five at the $300 price, five at $350, and 10 at $400. When you request the lowest fares for six seats, you’re automatically offered the $400 bucket for all six bookings, the lowest price at which six seats are available. The algorithms are not programmed to offer you five seats at $300 and one at $350.

“They’re forcing you into higher buckets,” explains Bob Harrell, an airline industry veteran and pricing analyst at Harrell Associates. “If the lower fare is not available at time of booking for all those seats, it automatically rolls over from one bucket to another.”

Harrell cites contributing factors; for one thing, there generally are fewer seats at the lowest fares. Also, there may be technical limitations when fares are loaded into global distribution systems, the reservations tools used by travel agencies, corporate travel departments, and online travel sites.

So why not reconfigure the algorithms? “The airlines may be saying, ‘We’re doing it because we can do it,’” Harrell says. If so, it wouldn’t be a first in airline pricing history.

We reached out to the airlines to explain this phenomenon. American, Delta, and Southwest confirmed the availability problem at the lowest fare levels; a Delta spokesman states: “I’m afraid I can’t get into a detailed discussion of inventory management out of proprietary and regulatory concerns, but I can express regret that you’re not seeing everything available at the same price point."

Related Stories

In our shopping tests, we used two different browsers to simultaneously search identical itineraries in real time on both domestic and international flights on four major airlines (American, Delta, Southwest, and United) and four popular third-party travel sites (Expedia, Google Flights, Kayak, and Trip Advisor). The first browser sought lowest economy class fares for one traveler, while the second did the same for the maximum number of travelers allowable (which can range between six and nine, depending on the airline or booking site).

In 16 different comparisons, we found higher per-passenger prices for multiple passengers nine times. The smallest differences in fares were on United, with just a $1 bump for a flight between Chicago and Denver. The biggest was the New York to L.A. trip on American, at an additional $103. There was a $94 difference on a Southwest flight between Washington, D.C., and Orlando, and a $37 difference on Delta between Atlanta and Dallas. Comparable differences turned up when we tried to book on the third-party sites as well.

Timing Matters, Too

In our shopping tests, we also alternated travel dates, between 14 and 90 days in advance.

We found that the pricing differential occurred when we searched three months in advance as well as two weeks prior, debunking the myth that the best deals must be purchased early. Bob Harrell wasn’t surprised by this. “I think it’s a misconception that lower fares are on longer booking windows,” he says. “The airlines really begin to manage the pricing only 30 to 45 days prior.”

Tips to Get the Lowest Fares

• Split your ticket purchase to take advantage of lower fares for some members of your party. You can comparison shop online—using two browsers is best—to see how many seats are available at the lowest fare; this technique also may help secure adjacent seat assignments. An American spokesman agrees: “The work-around is to buy individual tickets, so at least some folks can get the lower price."

• George Hobica, founder of airfarewatchdog.com, advises to consider transacting by phone, particularly for Delta, which no longer charges fees for calling. “Simply book seats two by two,” he suggests.

• Consider broadening your search. Flying on an adjacent day or at a nearby airport may yield lower fares, and many booking sites offer tools to assist with this.

• Consider the airlines’ group booking options. The four largest domestic carriers—American, Delta, Southwest, and United—offer dedicated group reservations via phone, email, or an online form, but only for parties of 10 or more. Southwest advises “groups receive access to special discounts off published fares.” But before accepting, first search for one traveler to obtain a base fare for comparison.