Weird isn't it - and you get the Sky version in full 1080i50 at a ridiculously high bitrate
iPlayer on Sky distribution is handled by Sky via their CDN I believe - so the BBC doesn't have to 'pay per bit' (so higher bitrates don't cost the BBC anything).
Sky, it seems, are keen for their Catch Up service to be decent quality it seems, with Sky content also usually having a Dolby 5.1 audio track if the linear transmission did.
All4 and ITV Hub on Sky are 1080i25 at a decent bitrate. On their web streaming versions they are 576p25 at a low bitrate (adverts are 360p25 - or were last time I checked)