fuel selector valve on a Cessna 172 is not a true, both/left/right
That is neither true, nor absolutely wrong. The fuel selector does indeed switch off the corresponding other lower tank outlet, so - "left" does close the right tank lower outlet and "right" does close the left tank lower outlet. BUT, due to the old crazy idea of probably Clyde Vernon Cessna, the overhead venting line, which is supposed to vent the right tank by the left tanks fuel cap and is not located above the full fuel notch, does create some cross feed when tanks are almost full. So, if you go full fuel, you may have, depending on model and sideways balance of the Cessna, several gallons to leak over the overhead venting line from one tank to the other.