What has happened to this plant?

Bought a watermelon plant from a local garden centre, and it seemed to be growing like a watermelon with the correct leaves. Picked it because it was ready and we achieved what I call a 'black pumpkin'. What has happened?

It had a field spot which is found on watermelons, unless pumpkins have this too? We are very new to gardening.

I have attached some of the photos.

 

Looks like wrong label , looks like a pumpkin to me as well.

 

It says grafted.
Just wondering if this is the rootstock?
Perhaps the graft failed and nobody noticed prior to sale.

 

I had to look up "field spot", seems it's the underside where it rested on the ground.

To my knowledge you get this in all the cucurbit family.

 

Watermelons are grafted onto squash (and gourds).

Their leaves are different. What were the leaves like, Watermelon or Pumpkin?