Occasionally, as rewards are confirmed and being processed from 'unconfirmed' to 'confirmed', they disappear for a short while from 'unconfirmed' (and therefore the total) before then appearing in 'confirmed' (and therefore the total). Seen this frequently!

Very occasionally, a block fails confirmation (invalid, orphaned?), and thus disappears from the 'unconfirmed' without obviously being added to the 'confirmed' a bit later. Seen this a couple of times!