The term "yellowbelly" is actually misleading. They don't typically have "yellow" bellies, and the color of their markings can be just as variable as it is in a normal ball python. They CAN have yellowish bellies, and I've always figured that the name came from that quirk of appearance in one of the founding stock, but more often than not, their bellies are creamy or white.
Yours is not a yellowbelly morph. But is a very pretty ball python, even so.