Be careful not to mistake your own experience as a universal rule.[/boringselfsatisfieddroningon]
Oh, it's been universal so far. Every 4-string bassist and every 6-string guitarist I've ever let play my 7 stringed instruments has this cognitive dissonance. Every one. Without fail. There have been, probably, close to a 100 people now. I feel pretty confident in my statement. As a matter of fact, I'm still waiting for a different reaction.