I think your math is off. As of the update there is a 10% chance for it to occur, which is irrelevant because that only determines if the pocket was targeted.
If a pickpocket occurs using your 12 slot setup of: 1 C4, 3 Milk, 8 Empty, the % math would be...
C4: 8.33%
Milk: 25%
Empty: 66.66%
One the system has decided that your pocket is the target, an item WILL drop at that point. It's not all of your pockets = 10%. Pretty sure of that.
Alternative way to explain it.
It's not 10% of the pickpocket chance divided by your slots. It's 10% chance to happen, which will then select one of your slots, assuming you have enough objects in the pocket to meet that limiter.
The two most surefire ways of avoiding a pickpocket from stealing your items, don't have more then a few items in your pocket, as if you have too few items it will not target your pocket. Either that or just be really poor, which is another criteria of avoiding pickpockets.
I suppose if you wanted to compound the math with the % of it occuring as well, you'd just shift the decimal one to the right making it:
C4: .83%
Milk: 2.5%
Empty: 6.66%