Why does the number of items in the address bar dropdown not match "maxRichResults"?

I've noticed that the Address Bar Dropdown list seems to be getting shorter and shorter. There are about 10(ish) sites I visit literally every day and originally all ten would appear in the dropdown when I clicked on it. However, they've been gradually dropping off the list, was down to seven last week, six yesterday and today there are only five items on it. At this rate in about a fortnight's time the list will be empty.

I've checked "browser.urlbar.maxRichResults" and it's been set to 12 the whole time. I tried bumping it higher and it makes no difference. If I set it lower eg: 3 then the dropdown is reduced appropriately. Setting it back to 12 again brings it back from 3 to 5.

So why the heck are these commonly visited sites disappearing from the dropdown? And how can I stop it happening?