Which of these batteries is actually low?

My family latched on to these battery testers a few years ago, clever little things that gauge the size of your battery and use that to figure out how much power to expect from them, so that they can compare that to how much power they are putting out, thus giving you a reading on how much “juice” is left in them.  Lots of batteries seem to have the same kind of thing built into them these days.  Whatever.

I’ve been noticing something recently: lots of products use multiple batteries (2, 4, whatever).  When the device starts to complain about its batteries being low, I take them out and test them.  Often – usually, I’d say – I find that only one of them is actually low.  Take out four batteries, three of them test perfectly fine, and the fourth is totally dead.  I have to suspect that this isn’t just because the one battery is failing, I think it’s something to do with the physics of batteries.  But, I don’t know what.  And, I could just be wrong.  But, anyone else experience this?

Whatever the case, you might think about checking all of your batteries instead of just throwing them all away when the device they are in says they are low.  Curious to hear if others experience this as well.