I play the guitar and sing for fun. I have some understanding of music theory but not much. I've been told a couple of times that I was singing the third. I also noticed that when I play around transposing chords (usually with a capo) to find the best vocal range for me I tend to sing the third (or some other note instead of the root/melody?). I wonder why I do that. I usually try to find a key that's good for me to sing, but then sometimes I realize I wasn't singing the right notes, nonetheless I think it sounds good.
Is it wrong? Why do I tend to do that? Any suggestions of how to find the right melody line on a different key?
As a reference, I'm a baritone.