I haven't seen these displays myself yet, but as an educated guess...
I would guess that the blindset has been updated and whoever now has responsibility for them at Arriva has found they can do it too. Can't comment on why a vehicle had one one day but not the next, though - perhaps the update was rescinded, or there are options available, one with, one without. (Although as 6402 likely has an aged blind, I'm surprised it could handle it to begin with).
Chris