At a guess a reason for separation would be 773, 77W, 77L, 77-8, 77-9 then throw 78-8/9 in. Then with small (<30 order) would expect a Max of 2 78 Sims so Recurrent Training bottleneck. Way too many differences to make viable safety case for MFF until Fleet Retirement leaves 77-8/9 and 78-8/9.