That all depends on the vendor. I've seen the same thing done in laptops, where one batch of a particular model might have a single CCFL backlight, and another might have a dual CCFL backlight, and neither are compatible with the other because they use different inverters. Having different LCD's in the same model laptop is very commonplace as well. Basically, if you want better quality control, then expect to pay more for your product.
This type of thing happens ALOT with Processors and Ram in particular. I know because I work with this in my Job.
**What happens is they get a batch of say processors that are to Run at say 1GHz, well btw: this is at the manufacturer level, not once they are sold already. Well, the main factor to that thing running at 1GHz is heat. SO say an entire batch runs thru Quality control testing and they are all failing at 750Ghz, any faster....failure.....
So, what happens-----They sell that entire batch as 500MHz Processors, the same goes for RAM. You can trust me when alot of people say, well just fan/etc. cool them better. But as I said, I am a Thermal Systems Design Engineer (Mechanicanical Engineer), and I'll just explain by saying that the heat is Generated deep inside the chip. Fans, water whatever is cooling from the outside, and it can't "Suck" the heat out of the middle of the chip fast enough. No matter how much cooling is done on the outter surface, the middle of the chip will overheat, there is no possible way to get the cooling to it in time.
So to get back on track. I would not doubt for a second that they use alot of parts like this, "Technically" they are new, but not up to original standard, But they are "GOOD ENOUGH"
Just a total wild guess, but it's done all the time, now as far as actually using left over spares. The only way I can see that happening is if they were able to buy 'lot sizes' large enough to produce a significant run of phones.
Just for their warranty and technical support alone, they want as much standardization as possible, so if someone was dumping a huge lot of this or that sure it's possible, otherwise, no way will they be buying a few of these here and some of those there....
Oh well that's my 2 cents, which is about what it's worth....
Have a great deay to all....Jeff