This is a great question, because I’ve wondered it myself for quite awhile. This gave me the chance to reach out there and see if I could get any more detail. Sure enough, I could. Let’s start with the question.
. . . I booked my tickets last week and got an interesting email from the online travel service this week that my return flights had been changed by the airline. The departure and arrival times were changed by 1-3 minutes, for the most part making the flights a bit longer. For example, my departure from Buffalo was changed from 4:30 to 4:27 (or it might have been the other way around).
Why would Delta make such small changes to the flights? I’m a private pilot and come from a family of pilots, so I’m familiar with airspeed versus ground speed, but there’s no way they can predict the weather nearly two months out; and I know about flying at maximum economical speed as opposed to maximum possible speed, so I’m wondering if the rising fuel prices have caused them to adjust their speeds a bit to save fuel. But that is just a wild guess.
Perry
This happens on all airlines, but I’ve certainly noticed it happening more frequently on Delta than others. You think your flights are all set and then you get a schedule change notice saying that the arrival time has moved by a minute or two. Why the heck are they doing this?
I went to Delta for the official response and was told this:
. . . when the schedule is first published, it’s not operationally sequenced, so small adjustments are made on certain routes – basically massaging the departure and arrival times slightly for the best operational efficiency and connectivity. As your reader noted, these are minor, a minute here and there.
In other words, when Delta first puts the schedule out there, it’s not perfectly timed to fit into the entire system. Little tweaks of a minute or two can make a difference. That sounds really nebulous, so let me try to fill in the blanks.
A schedule is put out there nearly a year in advance. Often, that won’t be the final expected schedule. 
Major tweaks happen along the way, but the big ones are usually done when you’re a few months out. There might be some new routes added since the schedule was first put out there. Some routes could be cut, frequencies could change, etc. So at a large airline like Delta, when the schedule firms up, they might find that tweaks of a minute or two on other routes can help.
There are a few other reasons I can think of that would make this worth doing. Look at it through the eyes of the reservation system. This is more of a small change than a big one, but let’s say that Delta flies from Buffalo to Tampa via Atlanta and it takes 4h32m. Then let’s say that US Airways can get you there in 4h30m. If there are a lot of people that fly that route, it might make sense for Delta to find a way to shave 3 minutes off the connection time. That could make it the fastest way to get from Buffalo to Tampa and it could actually have an impact on bookings. Of course, you only do this for larger markets.
There is also the issue of weather. Airlines use historical weather patterns to determine how long flights will take. For example, winds are much stronger in the winter than the summer, so a flight from JFK to LAX is scheduled to take longer in the winter. It’s possible that as weather data comes in, the airlines make slight changes to adjust to the new expectations.
The last reason I can think of for this is simply one of airport issues. Let’s say that Buffalo has construction going on this winter. Delta might want to lengthen the expected flight times to account for the potential ground delays, even if it’s just a few minutes.
I’m sure we have some readers who know more about this than I, so feel free to chime in below.
