<<why>>
<<not that I know of>>
If you watch the footage of them recording PRIDE you can see them discussing about the length of the song. They thought about it's success of the song as a single/video/radio.
They sped the BPM on the single versions of SIAMYCGOO and other songs released to radio. In the digital age this can be done without changing the pitch of Bono's voice - I don't think it could be done in the 80's.
I've also heard an unsubstantiated rumour from another knowledgable U2 that the entire ATYCLB was slightly sped up during the final mastering phase done by the record company. Considering the labels attitude towards U2 in 2000, the whack mastering on BOMB, and some outtake/MDH material I can find this easy to believe.
u2fp