Comment by maxerickson

I imagine that the variation is in the internal combustion engines the system is being paired with. In that scenario, it can be that the regulator is treating the combined units as a new drivetrain and requiring certification of each combination as if it were a new engine.

It would be interesting to see a breakdown of what larger operators have in their fleets. It could be that a few certifications go a long ways. They are going to be at least somewhat inclined to avoid variation.