yeah spotted that as well, its a ploy for the ultra hd channel, it goes like this, the first broadcast of ultra is at 29mbs bit rate so it looks gorgeous, then next one will drop to a lower bit rate but at the same time BT sport will decrease the HD bit rate so as to make ultra still look good. And by dropping bit rate on the HD channel its cheaper to run as the pay by mbs used.
pathetic but hey this is BT, sky have copped on and upped their bit rate so it looks ace.
Incidentally if sky and BT had fewer mickey mouse channels they could max out the bit rate on HD and there would be no need for ultra hd. In fact HD at 20mbs would knock spots off ultra.