What I've seen is something like; fleet's max is say 3000, you arrive at planet re-supply with say
1250. It would take 2 or more turns before it would again max out to 3000. I've seen this quite often in 1.41 and 1.49 and even in my hot seat games. It's not only for fleets but happens with single ships as well.
ps. what are you doing up so late, I thought I was the only one on, kids in bed and have some time to my self, but usually everyone on forum is already gone and in bed around this time
just some ideas mac