My understanding is that there is a combination of two factors at play here.
Firstly, FirstGroup needed all the trains in service faster than Hitachi were practically able to deliver them. As nice as a consistent fleet would be, their fleet plans relied upon new trains being delivered that quickly.
Secondly, CAF are supposedly determined to win orders to keep their factory going. As far as I understand it, both their TPE trains are based off existing but unimplemented designs. National Express had planned new InterCity LHCS for their ScotRail bid, presumably with a driving carriage and four intermediate carriages. FirstGroup had planned to use the electric 125mph InterCity design to replace the Voyagers in their ICWC bid. Since both products had already been designed, churning out a few would be a reasonable way to keep the factory going for another few years. It seems likely that there will be commonalities between the InterCity sets and the regional sets built for Northern, so it's not like they'll be totally unique on the network.