FWIW I had a Clancy Bros album in which one of them did an accapella song. Over the course of five or six verses, he drifted up about
1 1/2 notes from his starting point. I was amazed that they kept it on the LP.
So you may be in good company.
I, on the other hand, always sing precisely on pitch, and have never missed a note...cough, mutter.