The red needle has a little tab on it so that when the actual indicator rises, it pushes the tattle tale hand up as well. It is then supposed to show the peak reading when the line pressure goes back down for whatever reason. It should not go back down by itself, it should require you to turn the knob, otherwise, it would be useless for the tattle tale function.
If the red tattle tale hand is moving because of vibrations, I'd consider buying a new gauge...it shouldn't be that loose to turn by itself without the main needle pushing it up.
You wouldn't notice the WH T&P valve releasing (assuming it's working properly) until the pressure exceeded 150 psi, so in theory, unless that's sticky, the pressure should never exceed 150 because the T&P should open to relieve it. If it's sticky, it might peak a little higher, I suppose. Too sticky, and it should be replaced.
Follow Reach's suggestion, and verify that the T&P has not opened to discharge a small amount of water after using a moderate amount of hot. The water will expand when it is heated, and if the ET is working, the pressure should not rise much at all. If it isn't, depending on what else may leak (toilets are common and you wouldn't notice unless you heard it since it just goes down the drain), that water normally goes into the ET. Since water doesn't compress...expansion will spike the pressure unless there's something either leaking, or can balloon to take up the increased volume (ideally, that would be the ET).