It has more to do with the quality of the oil - vehicle manufacturers always have to err on the side of caution to allow for some people using cheap, poor quality oils. I recall 1955diesel saying that the oils that he used in testing the engines at Rover were a "worst cas escenario" - ie. pretty poor quality.
A lot of commercial users of vehicles now have the old engine oils tested, and have found that they have been able to extend service intervals by up to 50% in some cases because the oil was still perfectly serviceable beyond the vehicle manufacturers recommended change interval.
The reason for the reduced service interval on the Euro 6 MG3 will almost certainly be down to using a thinner oil, which (whilst improving fuel efficiency and therfore emissions by allowing the engine to turn more easily) will certainly not be able to offer the same protection against wear that the previously recommended oil would do.
Re: the PG1 - Rover initially filled them with 10W40, but subsequent problems and warranty issues led them to change first to a dedicated gear oil, and then to an extended service life gear oil, but still allowing 10W40 to be used for topping up. The biggest killer of gearboxes is lack of oil rather than the grade used though.