The most likely explanation here lies in your method. Please give us a rundown on how you install a string, step-by-step, including how you crimp the string before wrapping, how many wraps around the post, how fast you tighten the strings, etc.
As a guitar tech, i've tried several different ways of installing strings, and one of the things i've learned is that there doesn't seem to be an absolute best way of doing things, but it's important that you find a process and stick with it. This means crimping your strings the same way, beginning your string wind at the same length so that your number of wraps is the same every time, etc.
As was mentioned before, make sure your winds don't overlap each other. They should be neatly formed down the post. I like to have 3 complete winds on wound strings, and up to 5 on unwound strings. This is on standard tuning posts. I deal mostly with Spertzel locking tuners on tour, however, and these require only a single turn per post.
When i'm doing a thorough setup on guitars, one of the things i like to do is use various fine wire wheels with my Dremel to polish saddle slots, and the edges of the string holes on tuning posts. This greatly reduces the chances of string breakage due to burrs and friction.