favicons question

I don't understand how the Favicons get populated in a new tab page? I've got all of my Bookmarks synching across a Mac, iPad and iPhone and, although I've visited the pages on the Mac and the links now show the favicons, the iPhone still does not show all of them.

It shows some, and I've seen one update after a visit to the site in question, but this isn't the case for all of them.

I can see a pattern. Any ideas on why some won't show the favicon?