Explanations?
Basically the warranty states "Apple will either ... exchange the product with a product that is new or which has been manufactured from new or serviceable used parts and is at least functionally equivalent to the original product..."
Simply put they gave you the upgrade because it was available and it's possible the original part wasn't. By making the customer wait, they risk losing the customer; by compensating with a greater good, they entice the customer without making them wait, plus they get free advertising like this.
I've bought computers before and a lot of manufacturers do this. Usually you buy a computer and you wait 3 weeks, and then they tell you a part is on backorder? I call to cancel and I cancel and somehow it ships? Did they lie? No they didn't; instead of what I ordered, they gave me a better part. Which is worse, lose the whole order, or lose a few bucks in the profit?
Technically, if they give you an upgraded part and you go back for a replacement and they happen to have the equivalent of the original configuration in stock, they are allowed to "downgrade" your computer, because your configuration is attached to the serial number, not the repair upgrades. Same thing with buying; if you ordered 2GB RAM and they give you 4GB and then your RAM is defective, they legally only have to give you 2GB.