I just ran one of my searches, and Google showed me 7 results per page on page 4 and 5, then on page 6 it showed me 6 results. Back to 10 results on page 7, then 7 results on page 8 and 9. This is new? I've heard it mentioned for situations when sitelinks were shown, but none were being displayed in this case. I've never heard of only 6 results showing on a page.
Added: Just hit pages 13, 14, and 15 which each show only 5 results! Page 16 shows ten. I have never seen this before.