It’s an approximation of a triangle wave; spread over 21 values; with the same ascending/descending values (i.e. mirrored)
The values make a lot of sense; and having access to all hundred possible percentage values would not add much (42% vs 43%?) compared to the UI/UX overhead (i.e. all the scrolling through)
1 = 1/100th [99]
2 = 1/50th [98]
4 = 1/25th [96]
etc.
