How did you determine that the water was 10 degrees over temp? Were you relying on display of the the Anova or did you have an external thermometer? Reason I ask is that I have seen situations of the displayed temp being highly inaccurate due to a low water level. I've also seen them display complete nonsense in a failure situation. I would say to keep an eye on the unit for a bit and if it happens a second time, do not hesitate in sending it in for replacement or returning it and buying another unit.
As for the cookbook stating "use the smallest amount of water that will fit the food easily" - this is complete crap. With a small volume of water, you have lost your ability to buffer temperature changes, you've lost your insulation and circulation can suffer, and you have to constantly be on top of the water level as you have very little reserve when you lose volume to evaporation. Furthermore, when you add make up water to the smaller bath, something you wouldn't have to do with a larger bath, you'll have to match the water temp before adding or you'll either over shoot the temp or over cool the bath. For some foods this won't mater, for others, it could cause issues. Now, I'm not saying that you should use a 48-quart cooler to cook a single chicken breast, but something like a 5 gallon stock pot, which is what Anova recommends as the capacity of the cooker, is good for cooking most things. I use a 24 quart stock pot filled to about the max line on the Anova (probably right around the 4 gallon recommendation from the company) for probably 19 out 20 cooks and the machine performs flawlessly**.
**: I don't use the BT or Wi-Fi link since they suck and are prone to failure due to bad software. I know what my Anova's temp offset is, take that into account and just set the temp manually and use a ThermoWorks TimeStick timer.