A lot of people keep repeating this as if it was confirmed truth and
I'm pretty sure I started it with this post... but I don't know that this is true.
My theory -- just based on the fact that most people I know placed a little lower than last season ended -- was just that if the system has some wiggle range for estimated placement ("estimated 2600 to 2800"), it would go with the lower side of the range rather than high just because it does feel better to climb than fall. I can't imagine placements algorithms are so perfect that Blizzard gets a precise SR with a 100% confidence level out of them for everyone, and it made sense, based on Blizzard's past philosophies for releasing things in a state where later tuning will feel better for playerbase and on my anecdotal experience here and with friends on where people placed.
I wasn't trying to say that Overwatch intentionally shaves a bunch of points off of your SR just for a morale boost of winning a bit after, and I'm not sure if that is the case.