Exactly this ^. I don't think it's RL, PF or PL's style to promise a future release guarantee when they have no clue what his future value will be. If CB bags 30+ league and cup goals and has a great WC, he'll be worth £40m+ so why restrict ourselves to say £25m? It must have been a very persuasive meeting to get CB and his agent onside for a potential 4 years without a release clause. I expect £55k/week helped but when other suitors would offer similar or more, it's great work by the club.
EDIT: I can't help but think I've just jinxed us where CB bags barely a dozen. :-(