You said your name to the civil servant, and they might "misspell" it. That is: it's recorded "correctly" somewhere (e.g. birth certificate) but somewhere along the line it becomes "wrong" (registering your child, getting new papers, etc.)

My surname is Tournoij, which I inherited from my father. him, me, and my brother have that surname: the entire rest of the family is Tournoy. Mistake during registration when he was born ("y" and "ij" are often pronounced identical in Dutch), and apparently it's nigh-impossible to correct afterwards.

As to the original of Tourno(y|ij), my guess is that it's several degrees of misspellings from Tournai, a place in French Belgium.