For some reason, my average always starts out decent at the start of winter league, drops once the weather gets colder, and then I start shooting good sets again when spring time rolls around. And my summer leagues are consistently 10-15 pins higher than winter leagues
I am not an expert so take all of this with a grain of salt, but it could be:
1. Just a coincidence unless it is over several seasons, then it may more likely be one of...
2. Due to weather changes and its effect on the lane surface/condition. You may have to learn how to adjust to the changes in the lane conditions that you see during the colder weather. This may be especially true if you live in a colder or humid climate.
3. Physical changes in your body, especially the thumb. If your thumb changes because of the temp/humidity changes, you must use tape to compensate.
http://bowl4fun.com/ron/btm04_files/btm4.htmhttp://www.itbca.bowlingknowledge.info/index.php/2013-fall/95-tiered-taping-a-process-to--grip-pressure-and-improve-swing-releasehttp://www.bowlingthismonth.com/article/tiered-taping/4. Least likely, but it could be changes to the ball thumb hole due to weather. This may be due to keeping the ball in a car trunk before bowling or temp changes wherever the ball is stored between sets. The ball including the holes are expanding/shrinking or humidity is changing how the thumb exits.
5. The summer averages could also be higher because the center is using a different pattern than the winter leagues and it better matches your game. Or, you are more relaxed because it is a summer league and you don't take it as seriously as your winter leagues. If so, you need to work on your mental game.