This time their new system wouldn't accept figures for end of March to calculate the increase but would calculate them on an estimate, a whim, a magic guess using twigs & tea leafs..... who knows?
I believe it's supposed to be pro-rata, i.e. they asume that generation was split evenly across the time since the last readiing. Given the way generation increases during Spring that's bound to mean that any calculation for a range crossing the March 31st/April 1st tariff change will put more kWh on the lower rate than would have really been the case (assuming no equipment faullts complicating things).
Anyway it's not a lot of money but it is in their favour and if they do that with everyone then it'll mount up. Once the statement comes out I'll check it and maybe chase them up but worth watching your FIT payments for now.
Though AIUI they'll not be "pocketing the difference". It's not that sort of thing. Seems more like they can't be bothered to provide a way to get it right...