I don't know where the new Amazon USA date comes from - I heard it from someone else too.
The publisher is, obviously, in charge of the distribution. But the plan has always been to launch in the UK and USA at roughly the same time - with a USA a little delayed because of distribution - the UK Amazon launch date is 7th April (this week). The original Amazon USA (amazon.com) date was 27th April, which made sense to me.
I do know that we've all been very surprised by the demand and this has resulted in changes to distribution plans because there just aren't enough books to go around, maybe this is the reason for the new Amazon.com date - however, I still don't believe it, but I will update if I learn more.
Alex