Thanks for all the replies. Deep in my heart I always felt that the numbers must have had some type of meaning but could never understand what. Now I know they where meaning less
Yeah, there's no magical formula, but, in general, they are designed to give you an idea where the system fits into the product line. (IE, the A500 is a less expandable machine than the A2000. The A2500 is more powerful than the A2000. The A3000 is more powerful than either the 2000 or the 2500, etc...)
Then, later on, the A600 was incremented replacement to the A500. So the system that was a more powerful, but still self-contained unit was named the A1200.