Usually when I see or leave a 7-10 its because of a weaker hit.  The ball has actually begun to roll out and doesn't go through the pins properly, leaving a weak 10 with the 7 pin.  Very rarely do I see a "wrap" 7-10 because the ball hit too hard.  It's a bad break and hopefully the 5 or the head pin will get the 7, or sometimes you scout the 10 only leaving the 7.  Majority of the time its because the ball doesn't have enough energy going through the pin deck.

I agree with the above statements.  Most often I leave 7-10s due to a weaker hit.  Blower 7-10s I can't remember.

Though, I did leave a couple pretty solid 7-10s due to a off-spot 2 pin.  I was coming in from a deeper angle (18-7) and nothing got to the 7 while a messenger missed the 10.  Left 2 in one game.  Could have been a pin-spot issues also.

I have seen both! More usually it is because of loss of energy, however last night I got one that blew my mind because it most certainly was not from loss of energy. I set the ball down in the hold area, that had the most oil. The ball had good pace on it (faster than I normally throw it) with a decent amount of revs. It hit like a bomb! I just knew I had a strike,,,,but had a blower 7 pin and a wrap ten pin that just kept rocking but never fell!

Duct tape is like 'The Force'. It has a light side and a dark side, but it still holds the universe together.
Some days you're the bug; some days you're the windshield.

The majority of 7/10's that I have left in my lifetime are due to pulling the shot a board left into the carry down of the "second arrow" league bowlers and while the ball has plenty of hook and pace, it hits flat because the ball just never gets into a roll.

 

I've never thrown a 7/10 due to a ball burning out or losing energy, what does that look like in terms of pin action?