Leaven/ Levain Weight Discrepancy

Hey Friends, help me out here.

What the hell is the deal with the weight discrepancy between when you mix the leaven and when you add it to the dough.

for example:

20 g starter

100 g water

100 g flour

weighs 220 g

then suddenly the recipe only calls for 150 g of leaven.

PLEASE EXPLAIN!

why is there so much waste?