The fork exists on the plate... technically correct, but sounds weird in english and imagine it does in swedish as well. I imagine it is to do with the existence of the fork not being in question, but the location of it.
I didn't even get a choice, and did not know there was another way I should have said it, so I said Gaffeln på tallrikan. They are throwing new words that I do not know into the mix without context. Oh well, i am here to learn, right.