It's the discrepancy that's puzzling - surely it would make sense for the timespan to be too wide rather than too narrow (as has clearly happened with NM - the dates for the murder charge begin on the day of Becky's disappearance and end on the day her body was discovered, even though NM was being questioned by the police for the last few days of that and so he obviously didn't murder her then).
That's what makes me think there's some significance in the date of the 22nd, and that it must have to do with the movement of her body rather than them having proof of her death (because the latter would've affected the dates of NM's charge as well). The four charged with assisting an offender have no real reason to lie about the date when the bags/boxes were taken to the house, unless they're denying all knowledge which seems unlikely, so I wonder if the date comes from their own statements.