As said the 70% only applies to picking 2 position training when 1 position training is the default.
I don't know if the stated training speeds are accurate, at least not in all cases. For one, most training is height dependant, I think at least shorter players train faster in BH than displayed here. Also, not all skills are as valuable as the others, so that can be a reason to train BH too (because of the valuable OD it gives, albeit a little). Same goes for the IS in 1vs1 forwards.