alexist

Virgin tier bonuses are based on base mileage, not actual miles granted.

So effectively, you get the biggest tier premiums in LOWER classes. Compare:

N class economy, 1 way:
Red 1729 miles
Silver 2594 miles
Gold 3458 miles

Upper Class, one way:
Gold 8645 miles
Silver 7815 miles
Red 6916 miles

(Presumably, it prevents Gold Tier members from racking up the miles at obscene rates when they fly Upper Class, but it works out as a kind of perverse incentive!)