Interesting question, which seems pretty simple, and interesting responses. I'll share a recent story that will provide a different perspective.
I was in Cozumel a couple months ago. My party of four had asked for 8 nitrox tanks (2 each) for our dives on that particular day. I have my own O2 analyzer and while I usually don't bother bringing it with me when I travel, this time I did.
When we arrived at the dive shop, the tanks were there and ready to be loaded on the boat. The shop had already analyzed them and had labeled them all 32%. I pulled out my analyzer and started checking them for myself.
32... 32... 32... 21
Yup. One of the 8 tanks was air. How could this have possibly happened!?!? That's always the question immediately following an accident. Fortunately it didn't get to that point.
The shop guy was indignant and insisted something was wrong with my analyzer. I was equally indignant and told him something was wrong with him and his analyzer. And so began the pissing match.
He pulled his analyzer out (yes, his actual analyzer, not his...) and re-analyzed the tanks:
32... 32... 32... 32
He was ready to declare victory until I told him "recalibrate your analyzer and then measure that tank again."
He did. And his smug look disappeared. It read 21.
So what happened? I didn't tell him this, but just the week before I was in a shop at home and the same thing had happened to me: analyzing several nitrox tanks in a row, the readings were similar... just as I expected them to be. But some of my readings were slightly different than what the shop had measured. I pointed this out, and the shop guy told me that when analyzing multiple tanks in quick succession, residual gas from the previous tank (i.e. gas that gets trapped in the analyzer) can cause an inaccurate reading. He had me re-calibrate the analyzer between tanks (i.e. clear the residual gas from the previous tank) and remeasure. My readings then matched his.
The potential for mistake compounded by expectations: in the case of my Cozumel tanks, the shop guy expected every tank to be 32. So when he quickly moved his analyzer from one tank to the next, and he got 32 for each, he simply confirmed his expectations.
Two lessons here:
1. Be mindful of what your analyzer is really reading, i.e. the gas that is in contact with the sensor, and where it came from. If you have any doubt, re-calibrate.
2. Analyze your own gas. It's a standard rule that's hammered on in every nitrox class (or at least should be), but some places (like the shop I went to in Coz) just assume they are always right and encourage you to trust them (because, after all, you're on vacation!) But if you get bent from diving on 21 when you thought you had 32... the most you'll get from them is an apology.