Because things are being learned as we discuss them and as more analysis is done. If you're terribly worried about it, they're very close up to 490ish (485-495) and you can't really go wrong either way.
im going to make the assumption that something is wonky with simcraft for it to be doing this. There is no way you can have so much variance. Its all mathematical equations the game does and theres really only one right answer. So something is either not working right, or its assuming some serious RNG.