You're right, the pressure gauge shouldn't change without a leak or temperature change (Boyle's Law and such). But it's hard to have a 100%, no-leak extinguisher. There's plenty of points of failure to keep the thing pressurized, and the leak could be imperceptible (over months) or a rapid failure

And you wanna keep contents from settling as well, so you don't just spray gas when trying to use it. So you mix it a bit and inspect the gauge every so often, because you don't want it to fail when you need it most

Ideally, it shouldn't lose pressure. But also ideally, you don't have a fire that requires an extinguisher in the first place

But yes, you're right on that one. "Settling" of the fire-retardent wouldn't cause a pressure change. It's just part of the safety inspection to check the gauge as well as to "mix" it... keeping it ready to go