I've also had the experience of sitting in a singaround when the next person to sing was known to have a lot of interest in ballads but no great voice (in a technical sense). My heart was preparing to sink when my eyes were snapped open by a performance that nailed the story -- and I know that I wasn't the only person in the room to be taken by surprise.
So, what was the difference on that occasion? I've thought about this often and the best answer I can offer is that in that particular singaround the singer in question had set self aside; earlier performances by the same person had possibly required the audience to like the singer before liking the song.
Does that ring any bells with anyone else?