It's because you didn't account for the fact that you still need enough string to reach the back of the gripper (when pulling those final main, or cross, strings) and enough to tie your knots.
@Tennis sprew,
Besides hoop size, # of mains and # of crosses, there are a few other factors to take into account.
- (quite important) The type of machine you're using (distance from the mounted frame's grommets to the back of your tensioner. Rotational grippers require more string length to get to the back of the gripper than a linear gripper does)
- (quite important) Whether you're stringing 1pc. or 2pc. (2pc. uses more string than 1pc. does. However, learn 2pc. first - since you're a beginner)
- (fairly important) The type of string being used (some stretch more than others)
- (less important) The tension being used (the higher the tension, the more the stretch)
It's likely that you could use 18 feet for the mains and 17.5 feet for the crosses (or even less).
However, the smart thing to do is this...
Cut exactly 19 feet for your mains.
Once you are pulling tension on each of the 8th/outer mains, carefully measure exactly how much extra you have protruding beyond the back of your gripper. Subtract that (extra) amount from the 19 feet that you started with. Now you know what length that you
actually needed for your mains.
Cut exactly 18 feet for your crosses.
Since you begin with the 1st/top cross, you shouldn't have any extra length at the head of the frame.
Once you are pulling tension on the 19th/bottom cross, carefully measure exactly how much extra you have protruding beyond the back of your gripper. Subtract that (extra) amount from the 18 feet that you started with. Now you know what length that you
actually needed for your crosses.
Write those measurements down somewhere. Better yet, start keeping a "stringing log".