Tolerance works two ways, so the difference could be 1.8m.

However, I would expect that there is a high correlation between the size lengths of lanes in a single pool, so that one doesn't have one lane 3 centimeters too long and the one next to be 3 centimeters too short. It's weird that the standard doesn't make any requirements on that.

Also, touch plates get installed after the pool is constructed. I would expect those could easily be made to compensate for any size differences. That would leave a size difference at the start, but that's easily fixed, too.

Having said that, every few years or so there's a newspaper article of a swimming pool that gets constructed to be 50m, where the constructors forget about those touch plates.