You should note that the upload speed may vary as to how many peers you are uploading to. I’m guessing that the first torrent, with high priority, simply has a lower number of peers, or peers that can accept uploads only at a lower speed than the second torrent, which in turn makes the upload speed of the second torrent faster than the first.
but i limited the upload rate for the one on -low- to stay on 10, the other one -high- went up to 30, the priority thing should do this on its on, somehow..