[Thor]

ok, lets put it in the endurance 101 pics for dummies then

You have level 20 endurance, you buy 2000 credits and use them to up your endurance. You will get more dev points per train even if you're not level 21 yet.

Every credits invested in endurance gets you better training, it just doesn't show in the early levels because you usually have low endurance + low energy + a low/average happiness/house.

Simplest example:
level 50
110 energy
Beach house 1150 happiness
Level 30 endurance with 0.00 dev pts into level 31
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You train all your energy and get 280 dev points.
Donate 20$, use the 2300 credits to convert into 560 endurance dev pts.

You train again 110 energy with 1150 happiness and 560 dev pts into level 31 endurance and you will get 281-282 dev points now.

How's that for you?