Because of what i'm finding in the fuel bowl.
Small black flakes.
I cleaned it all out drove for just half a tank, and it had refilled with the flakes again.
We are talking about like a 1/4" of an inch in the bottom of the bowl.

 

I had the same. I just started running a can of seafoam in each tank for about 2 months and three filters and three clanings and now its all cleared up. Now I am running Marvel Mystery Oil mixed in when I tank up.

Some of my flakes were from the bowel and the bowl heater delaminating.

::Marvel Mystery Oil::

 

It also shows in that chart that the 22 gal tank in question is a plastic tank and not steel.

 

Hmmm, interesting. I didn't notice that.
I'll have to look at my tank. I know i've put over 20 gallons in it before.