Probably because of the karma difference between purple and red.
it goes like:
White
15 karma
Yellow
15 karma
Orange
30 karma
Light Green
30 karma
Green
30 karma
Blue
60 karma
Purple
20 karma
Red
40 karma
Brown
110 karma
Black
The large difference between Blue and Purple and the short difference between Purple and Red is weird
it goes like:
White
15 karma
Yellow
15 karma
Orange
30 karma
Light Green
30 karma
Green
30 karma
Blue
60 karma
Purple
20 karma
Red
40 karma
Brown
110 karma
Black
The large difference between Blue and Purple and the short difference between Purple and Red is weird