A-Ha! I posted to a network blog, and my tags blog made 8 copies of one of the categories.
This particular post was assigned 9 categories and only the last category was replicated...8 replicas with 0 posts and one with the new post in it.
Update: I was able to repeat it. It happens when a post has more than one category assigned to it. This time I assigned 4 categories to the post, and only the last one was replicated, and only once.
Notes: doesn't seem to replicate categories where the name starts with a number? I have 10th grade, 11th grade, etc...these don't get replicated. If I assign 2 categories and both are non-numerical...they both get replicated.
Conclusion I'm drawing: When there is more than one category assigned to a post, each category will be replicated...the number of times depending on the number of other categories assigned to that post. Numeric categories are not replicated.
I hope this helps in some way! Email me at email@example.com if you want me to try and explain better...