It's possible you got a 2 for 1 but it'll take several years before you can be sure.

I didn't think that the seedlings are related to the understock.

The seedling might resemble the parent tree but it wouldn't be identical. You really don't know what you will get with seedlings until they grow up. I've got a Bloodgood seedling that someone gave me when it was only 4" tall. It has turned out to be a nice tree with leaves that stay red all summer long, but I have no idea how all it will get or whether it will be as full as a grafted Bloodgood.

I would carefully dig up your seedling and transplant it to another pot. Let it grow and see how it turns out. Might be a very nice tree eventually.

Probably a seedling from another tree rather than the grafted dwarf, which in a one gallon pot is too small to be likely to be producing fruits. This cultivar has leaves similar to a normal wild-type green Japanese maple, hence the resemblance.