I can tell(most of the time) when im going to leave a 10 pin, but when I leave a 7-10 its off a shot that just feels GOOD. As inverted said, most of the time it comes from the blow out strike where the 5 slides the 7 pin out, but fails.
Ive been told its from too much angle? Maybe I miss heard. And as for the 7-9. Been there, done that. Its more insulting then the 7-10.
The only time ive spared the 7-10 was from a pocket leave. It was in practice, so I just hurled the ball down the lane as fast as I could, low and behold out pops the 10 pin and down goes the 7. The PERFECT revenge
good luck and high scoring
matt