It's not randomly. In work I use Iceweasel for private gmail (and Chrome for company gmail), where I have set Results per page to 100 and chose to Never show Instant results long time ago. It simply does not work!

I go to google.com, start typing and Google starts showing me some instant results and hitting Enter gives me only 10 results per page. (The same settings are respected for non-private gmail in Chrome, though.)