Admob balance request ads vs shown ads

Hello, can someone tell me how to balance ads request and ads shows?
For example only 20-25% of ads are beein shown, in admob i have only 20 ads shown from 100 requests, so, when i request an ad? at load reward video an add is requested? or the ads are requested at show video/banner/interstitial?
Also it is normal to have some difference between requested ads and shown ads?
Thanks.