actually, you are going about it the wrong way
if a glass has a volume of 2, then there are 3 states the glass can be in:
0 = empty
1 = the word we're getting to
2= full
But this is only when you are looking at it from your perspective, namely that full is the maximum... For pessimists, the maximum is not full, it is empty, so you get the other way around...
0 = full
1 = our word
2 = empty
now if you are going about the half-full, then you are measuring from the bottom up, since completely full would be the top of the glass.
therefore, full would be your maximum equalling 2
when you go about your calculations, you are correct in pointing out 1 or half-full as your answer, however, the flipside starts with empty as their maximum, which means for them it is 2, instead of full.
So they get to half-empty and have the same value for it, and consequently feel let-down already cuz it's already half-empty