That wouldn't be it, methinks. In all likelihood, the Pokemon are identified to the game in binary. This means that the limits for how many could be given would be based upon an exponent of 2 - the nearest one would be 512, so 493 (or 500) doesn't approach the limit. And if we're counting Unown and other special forms, then it actually clears the threshhold anyway, so adding more would be irrelevant. In terms of coding, it just doesn't add up.