Question A. Ever do a search in Google with the default 10 results shown on a page, and then switch the default to something else? If yes, then move to question B.
Question B. Did you ever notice that ones rankings can change a few placements from depending on the requested number of results per page?
A thread at WebmasterWorld discusses just that. valeyard in msg # 3 sums this up perfectly, and GoogleGuy in msg # 5 backs these statements up.
Example: let's say you do a search with "num=10". The results on page one are:
Site A Site B Site C - page 1 ...Site C - page 2 Site D - page 1 ...Site D - page 2 Site E Site F YOUR SITE Site G
So your site is #9.
Now imagine setting num=5. The first page is now:
Site A Site B Site C - page 1 ...Site C - page 2 Site D - page 1
It makes no sense to have the second page from site D indented at the top of page two, so page 2 is:
Site E Site F YOUR SITE Site G Site H
Hey presto! Your site has jumped to position 8.
The lower the value of "num" the more chance of indented pages being cut off like this - which seems to fit what you're seeing.