Side pockets open in the exact spot your stuff naturally rests when you sit down.
Shallow pockets force your stuff to the opening of the pocket when you sit down.
Horizontally cut pockets angle away from where your stuff naturally sits, keeping it nice and secure.
Deep pockets move your stuff toward the bottom of the pocket when you sit down, keeping your stuff away from the opening of the pocket.
We started experimenting with zippers early on when designing our shorts, but it was too annoying to constantly zip and unzip when pulling out our phone 1000x times per day. We noticed that people tend to open their pocket and not re-zip them closed which caused them to still experience the problem.
We wanted to make sure that your stuff was always within your reach, so we measured where most peoples hands fall by their side and found that the average persons hands lie 11 inches below their waist.
We tried velcro after using zippers, but it was too rough to touch and people couldn't stand the noise it made every time they opened their pocket. To make matters worse.. velcro often would close the pocket when people wouldn't want it closed and would require two hands to get their stuff.
The reason stuff doesn't stay in most athletic short pockets is actually quite simple, the vertically cut (or side) pockets open in the exact spot your stuff naturally sits giving it a path to the floor. Keap's horizontally cut pockets angle away from where your stuff naturally sits, keeping it nice and secure when you sit down.