Scott Irving is right and not worth worrying to much about it. 30uf is roughly the norm for 1hp motors, it's more about what you can get as a lot of intermediate sizes are not available to keep manufacturing costs down. You can always go to the nearest highest value without much problem. A lot of small garage machinery uses between 1/2hp and 3/4hp motors (bench drills) so 12 -15 becomes a reasonable rule. If your motor requires a greater start torque to get going then a smaller cap will not start it and you would need the next one up. I have an old 3/4hp motor that runs quite happily on an 8uf cap (go figure that), it has very high spec bearings and not a lot of resistance to get started so only needs a small kick. This i suspect is the manufactures being a bit frugal with costs and opted for the lowest value they could get away with?