Same thing happened to me (different #, of course) when I popped a previously used CF, which I did not reformat at the time, into my 5D. Also, I had a strange folder name, not what was listed in the manual.
I was puzzled and posted here and someone answered that it was probably the CF card. I reformatted the card, and then reset the numbering and Voila! I had the folder named in the manual and my file numbering started (at 101, I believe).