It's the twist of the thread when they load the spool. Weird huh?
Edited to add:
I read some where that the thread that is wound right next to itself should be on a spool pin that is vertical and the thread that is wound on the spool criss-crossing should be on a horizontal spool pin. This helps with the thread not being twisted tight as it comes off the spool. Some LA's have a problem when their thread twists and they have a lot of breakage/tension issues.
Also read that if you 'drape' your thread (about 10 inches) and it twists together it is poor quality and will cause all kinds of problems. If it stays in the U shape it is good quality.....