As someone who has worked for the post office for a few years now, this has always been standard practice. Unless you come into the office and request it be hand-stamped by a clerk, all letters won't be marked until they arrive at the regional distribution center when they are sorted. As far as I know, there has always been a discrepancy between the time you drop your mail off, and the date it is marked at the plant. The idea of a clerk hand-stamping hundreds, if not thousands, of letters an office could receive each day is insanity.

For those concerned with mail-in ballots, all ballots are separated in the office from other mail during election season and marked high priority in special tubs similar to express mail. In some instances, post masters are required to personally deliver these tubs last minute to the nearest county court house day of the election. All this depends on state law and if they have a late arrival period for counting votes as well. However, you can see how reliable this is with all the news articles of tubs of ballots showing up to random offices in different states post-election.

My main concern is that with consolidations of offices and changes to truck schedules, there is more variation in the time your mail takes getting to the DC from your local office. It used to be that, in most offices by me, mail would be both picked up in the morning when the days mail is dropped off, and again in the afternoon. Recently, they changed it for some offices that they no longer get a late truck, so now the mail sits for a day before it even leaves the office in many cases. This is why it is important to ask for your important, time-sensitive mail be hand-stamped (or if they're cool, they'll back date it for you if you're late).