Two reasons for this

1) They've been sat in the same place for a couple of weeks - and they've already taken a full 360 pan in L256 - in terms of high-volume data, there's not much more to be done where they currently sit, and the small Hazcam movies are hardly a data behemoth. They've also done quite a few sols of photometric studies (L2,4,7) at different times of the day. So basically, there's not much data they can produce and as a result, at each UHF pass, the downlink includes files of lower and lower priority until we get to images taken right at the beginning of the mission that were set as very low priority at the time

2) They're expecting to do a lot of remote sensing on 'Opportunity's Bog' once they're out - to characterise why this dune is different to the rest in the area. That will require lots of flash, so best to empty it out before you start :)

Doug