I had that with a recent order for a compact and memory cards. I seperated the order into two (camera on one, memory cards on the other). The camera was accepted for next working day under my Prime trial, the memory cards showed a significant delay and a delivery window sililar to the one you mention (despite showing as in-stock). Within an hour of placing the order there was an email update to say that the memory card order would be shipping sooner than expected. Both orders arrived within an hour of each other (camera by RM parcels, memory cards by regular RM) next working day.
If you have multiple catetgories of item on the same order the Amazon system seems to go all cautious and super-conservative on the delivery dates.