It's based on an 'average' of what works for maintaining a bankroll without facing bankruptcy taking swings and variance into account.
2% on MTT's. - Large fields, % of times ITM will be lower, requiring more MTT's played to stay afloat.
5% SnG's - Smaller fields, a few 180-360, but mostly 90 players or less. You should be able to make it ITM a higher % of the time.
100BB - no limit ring games, the 100x is to buy-in to ONE session. That does not mean you enter one session with 100BB as your total BR.
50BB is the standard top end of buy-ins for limit ring games. Again, does not mean that is your total BR.