League scoring is weird... it works out in the end after a session of many (20+) weeks, but in that time even the top players will have horrendous nights where the rolls don't go their way and/or they miss a shot they really should make.
It seems I always have a slump during our league (27 weeks/5 games a week) where I'll get below average scores for spurts and they shoot lights out for a few weeks. On one occasion, we had to make up a week that was cancelled and I, playing with a 17 point system (5gamesx17point=85points) shot a 27 and a 73 on the same night. One of my worst, followed by one of my best. Came down to rolls.... good or bad.
You need a totally different approach to playing league compared to play a race to 5 or 7 in 8 ball. League has no real flow and you have to treat every game like your hill-hill in a match, where in a match, you can sometimes relax a bit if you're up a few games and feel like you're shooting well.