If the result somewhere is Bernie 30%, Biden 25%, Warren 20%, Pete 15% — that means, Bernie gets 1/3 of the delegates.
But if it's Bernie 30%, Biden 20%, everyone else sub-15% — Bernie gets 3/5 of the delegates.
So you clear out everyone below 15%. Then your share of what's left will be your share of the delegates.
But in the first, the viable candidates' vote share is larger (90% when added together). So 30% is only 1/3 of that.
In the second, the viable candidates have just 50%. 30% is 3/5 of that. Much bigger haul