Why is it a highscore and not highestscore?
I mean how can a low score be a highscore, while a high score does not have to be a highscore...

Because a highscore list does not only contain the highest score.

It should be topscore.
Fits both single highest score and list of sorted high scores

@ViRaS and also golf situations where lower scores are better.

@Olverine Yes

@Olverine They are better, but still lower. So you win by having a lowscore, not a highscore

@WIPocket Yes but a 'topscore' could also be the lowest score.

Are you high?

@ViRaS What if it was a game where you win if you sink to the bottom and lose if you float on the top. Would topscore mean the best or worst score?

@electrineer Topscore means the best score. It can either be a low score or a high score depending on the game.
So for a game where high scores are better, the list will show something like: 100, 80, 90, ...
And for a game which prefers low scores, it will be like: 10, 20, 30, ...

To be honest it should simply be the best score.

@Santaclauze That'll work too