That's not PC whatever relationship, that's just reality of delayed exclusives regardless where they get ported to. Look back at PS2 and its LTD best sellers - outside for the lone 1st party game or two, nothing in top 30 is exclusive.
There is a fair number of PC-360 games that co-launched and in almost every case the 360 version dwarfs the PC version. There are exceptions--and delays really dent sales even more--but I don't think there is any question that in "Console Compatible" titles (i.e. co-developed or whatever you want to call it, but titles aimed to straddle both demographics) do a lot better overall on the 360 regardless of release date.