Believe that fact that the electronics in the XC and NX varities were replaced, thus different and hence number change.
With the exception of 43053/056 on GW.
I'm sure 43034 The Black Horse will be able to shed more light...
GNER/NXEC, XC and NMT power cars had electronics upgrades with new cubicles. Mainly a new WSP System. Whilst in the process of transition to MTU from Valenta, powercars would have needed Exams. And by renumbering them, people would know what engine was fitted to which without having to go and look at them of knowing what was Valenta or MTU off by heart.
E.G If 43313 required new electrical parts, you know which ones to order as it was MTU.
If 43113 needed new electrical parts, you would either have to go and check the Power Car or know off by heart which PCs were MTU and which were Valenta.