Speed doesn't reach upload limit, but setting it to unlimited gets max

[flyz]

I'm using 1.8.1. I can't seem to upload at the limit that I set it to, but if I set the upload limit to unlimited, then it gets full upload speed, usually. For example, if I set the upload limit to 50Kb, it will usually only upload to the peer at 15-25KB, but then if I change it to unlimited it will upload at max which is like 70-80Kb. I notice this when I am the sole seeder for a torrent. I have tried checking 'limit local peer bandwidth' and unchecking Automatic limiting but it doesn't help.

I have tested it out repeatedly. I set it to a certain limit and it uploads 1/4-1/3 what I set it to. I change it to unlimited and it goes full upload speed. I change it back to a limit and it goes below the limit, change it back to unlimited and it goes back up. Why does it do this?