That's what happens when you charge using whole cans. Taking pressure readings will never do that if you don't put too much in. Problem is, everyone doesn't want to pay for the work and would rather charge it themselves, then they refuse to discard the partial can (money) when the system uses only part of it, as they all do if charged correctly.
I always stage a charging party, or more than one car to be topped up at once to save that at least once. I use gauges so I always end up with a partial can just to let it go to waste. The nature of the beast.