If you pour the maximum amount of water that fits inside, then your coffee will brew close to the boiling point of water, which is too hot and makes the coffee bitter.

If you pour less amount of water, then the coffee brews at a lower temperature.

The principle is that the coffee is pushed out by both the steam and the expanding trapped air inside the moka pot. The more expanding trapped air you have there, the less pressure you need from steam, so the coffee starts brewing earlier.

I've tried different recipes, different beans, etc. Right now I'm on Illy espresso (got some on sale).

I hear your suggestions :-). The beans in Italy were basically beans in a tin, open to the air. But it was Italian air, Venetian salt seasoned air, and I was on vacation. Could've been that.