NIS 2011: On scanning few if any files are skipped?

As of today, I have upgraded four systems from NIS09 to NIS11.

Interestingly, the original system where I installed it a week ago is the subject of this post.

I find that on the three systems which I installed today, after the first full system scan, subsequent scans skipped 50-90% of the total files on the system.

However, on the original system repeated rescans have hardly any skipped files at all.

Anyone know why that is?

Thanks.