Strong grip, no flip - don't understand

I have a bad habit of swinging too much on the inside with short iron shots then flipping the club face at impact, resulting, sometimes, in shots that finish well left of target. I thought back to a lesson I had a while ago when the pro told me to use a stronger grip. I tried this at the range & most of the shots went much straighter. Can't understand why. I would have thought that a strong grip would have sent the ball further left.

Strange, when I try the same grip with a 5 iron everything goes well left.

Anyone got an explanation?