I've been experimenting with the same motor and have noticed the exact same phenomenon. It's commonly reported that these are 32 step motors with 64:1 gearing, but I have read another post were somebody opened up the motor and counted the teeth on the gears. They reported (and I believe it based on my results) that the gearing ratio is not an integer. This makes it impossibe (or at least exceedingly difficult) to use these motors as an sort of clock motor. You can certainly use them in back-and-forth type of applications - they are intended to open and close ductwork.
edit: I found the discussion I was referencing above - read about halfway through the comments.
http://letsmakerobots.com/node/31381