If you use a Vertex Group to select a part of your mesh to decimate, the Ratio gets ignored. 
EDIT: Turns out it doesn’t get ignored, but a ratio of 0.02 without a vertex group produces a very similar result as a ratio of 0.8 with a vertex group. 
The devs care so little about UX that they didn’t even update the Ratio slider with the total of the vertex group instead of the entire mesh. 