What do you think floats on the top?

Dirt? Heavier than gas.

Water? Heavier than gas.

Oil? Heavier than gas, and would actually mix with the gas.

I have never seen any "junk" floating on gas. All the junk I have ever seen was a layer at the bottom of gas cans.

The hesitation is probably some air being introduced into the fuel supply from running very low and the fuel moving to the back of the tank and uncovering the fuel pickup.