How is that clearer?? :-)

In fact, it's not even the same thing. A 100-gallon pool (for very small people) that is 99% full has 99 gallons. 98% full has 98 gallons...

99% water by volume to 98% water by volume. or another way, sediments increasing from 1% to 2% would make sense if half the water evaporated

Not 99% full, 99% water. Presumably the pool contains a solution of some sort.

Hmm how about: You have a 1 liter bottle of 99% alcohol. How much of the alcohol you should evaporate to have 98% alcohol left?