Just a hunch mind you, but at my house, that Delta faucet single-lever is always parked in the middle position. When any of us need water we push it to open, but I seldom see it then turned to the right which would give us only cold water. Now, if we fill a pot of water, we do push it to the right, but initially 'some' hot water will be released as the valve is opened.
I felt the hotwater tank outlet line heat up as the water was drawn with the valve 'in the middle'.
Add up all the single lever taps on the planet, and the bad habit of not turning the valve handle to the 'cold' position, and you got yourself another source of wasted energy that can only lead to the oceans going up another foot.
I say: OUTLAW single lever taps.