There are enough reasonable differences between the Su-25 variants whereas there is little between GR.7 and 5. This will also be the UKs first jet with AGMs (the main reason for it being added). USSR also already has the MiG-27M/K and Su-17M4/Su-22M3 for other top CAS. The Su-25 early also suits the Rank VI gap in the tree where intended. The situations don't compare.