I swim in mine a lot. I notice the same thing but that extra push off isn't necessary. It's smart enough to calibrate itself. When you hit save, it assumes you are at the wall and it will calibrate itself to the nearest length.
Tonight for example, I swam 60 lengths, or 1,500 M. I looked at the watch when I hit the wall it displayed 59 lengths and 1,492 M. I ended the workout and it saved it at 60 lengths, and 1,500 M.
You've told it the pool length and it calibrates to a pool length when you end it. It's working normal, no need to do an extra push off. Your watch is working perfectly normal.
I'll try next time - it did work initially, but then one time it only registered 63 out of 64 laps which was when i started looking at what was going on. As the length counter seems to "lag" (i could check every now and again when the length counter went up) I lost confidence and did the extra push off!