In my experience of fretless playing, the meat of your finger needs to be behind the fretline, so the side of the fretting finger meets the fretline. Because fingers are quite fat, if you play directly over the the fretline, then part of your fat finger will be behind the line and some past the line, and will make the note sharp.
You will also find that the further up the neck you play, this finger position will change slightly, due to the physics of fretless.