You're right. It's not a watermelon. Seeds get mixed up sometimes, just as the labels get misplaced on potted plants sometimes. We do not recognize the melon.
Does it taste good? Then there is a good chance it may be a desirable variety. Do a search on 'white melons with white flesh' and you'll find many possibilities. Match yours with what you see.
This is not necessarily any particular variety of melon however. Melons with cross pollinate with another melon, squash or gourd and then you get a mixed up hybrid that may be useless.