In fairness to TITP's organisers, they've apparently applied to increase the capacity from 69,000 to 80,000, which explains the possible release of extra tickets. Same thing happened last year when they were granted an increase from 60,000 to 69,000.
I've no idea what the thinking was in holding back Radiohead tickets however. Not that I'm complaining as the evil c'nts at Ticketmaster sent me an email forewarning me of this so I could pick up tickets in the second release.