[mikmak]

That may be the reason: higher potential work load through increased RPM during low range engagement. This, in concert with a higher thermal environment (less cooling when you are going slow) would have to have a significant effect on MTBF (Mean time before failure) calculations. All servicings are based on worst case scenarios, then tweaked by bean counters against an acceptable number of failures within warranty periods. Just another cost benefit analysis where people argue incessantly about the value of having a good reputation and get shouted down by the boss who writes the cheques.