Actually, I did some testing and it doesn't only happen in the "All" feed list, but also in individual feed lists.

It's not very deterministic, but it seems to be related to items with "rich content" (pictures, graphic menus, comments etc.), maybe because they're slower to load ? Over-the-top feeds like Gizmodo, Engadget, Slashdot are very frequently affected, while plain, bare-bone, old-style RSS feeds like Delicious and personal blogs are almost immune...