Ah, I don't track every delegation separately so in your example, after delegating 60 SP, your share will increase and by this your curation rewards - so you will receive curation rewards like you've delegated 60 sp since day one. But, the actual curation rewards for @tipU will be increased after next 7 days (payout delay) and by this your's too.

For 40->60 sp that's not a big problem, but if someone would delegate 1 sp, wait 7 days and then delegate 100k sp - that would be a problem :)