So the real release date is 3/2 but they already sold their initial stock so the later date is when they expect more to arrive?
Pretty much, but, again, they can get stock before that date.
It is all logistics, depends on when the ship/plane arrives, trucks delivering them, and so on.
If you place a order on both, you may get both boards at the same time.
Very hard to tell who gets their stuff first, Amazon is bigger, and have more warehouses, and maybe even more customers, newegg is usually a priority as well so depending on where in the queue you are...
Edit*
If you want to believe rumors, supposedly, they only had 500 mobos, and 400 of them went to business orders, but CPUs are now shipping. That means, they had 100 left for consumers.
That sounds like a really low number though, not sure I buy that rumor.