In the 16 years I've worked for UPS, the start time for Local Sort (a.k.a. Reload, Twilight, etc.) has always been much later on Friday than that of the rest of the week. This is because volume is significantly lighter for a multitude of reasons: many businesses are closed Friday, people work fewer hours (and shipping is not a priority), people tend to defer shipments until the weekend/Monday (for example, on Friday, that college student isn't worried about getting his Chegg book returned-he'll do it on Monday), etc. Conversely, volume is heaviest on Monday, as people return to work + play catch-up. The first half of my career, Local Sort posted a weekly start time and employees knew to come in an hour earlier on Monday and hour later on Friday. Then UPS really started to stagger start times (4:30PM M, 5:00 T, 4:45 W, 5:15 R, 6:30 friend) and you had to write them down to keep track.
It might seem that way, but it's not. Unless your facility handles something like an Amazon distribution center, volume will always be lighter on Fridays (because shipping packages is just not a priority for most people on Friday). The two biggest inputs in production are labor hours + volume processed; if volume drops, you must cut labor hours to keep production numbers up. While it may seem like there's "the same people" working the same number of hours, I can virtually guarantee you there's not. YOU may be working the same number of hours, but some of your co-workers probably have Fridays off, while others are going home after an hour or two. For years, I thought the same thing as you until I was mature enough to understand otherwise.