What is the peculiarity of some torrent clients?

Question

bittorrent

In general, the story is this: there is an operator that has an unlimited tariff with a speed limit of 128 kbps. But an amazing bug was found, thanks to which the speed in some torrents (most likely utp) was not limited. But in contrast to the speed without restrictions when downloading from several nodes, the speed when a certain node arrives, and it usually gives a speed of 70-200 kb / s.
So the surprising thing is that this feature only worked in the Transmission and Qbittorrent clients. And regardless of the platform (both windows and Ubuntu). Although the effect is not observed in utorrent.
So the question is what is special about these clients that is not in utorrent and other downloads?