Decimation glitch for ratio < 0.2 I'm having trouble with decimation.

I have a mesh of a head. I wish to decimate to 10%, whilst keeping the ears at 100%.

So I create a Vertex group for each ear, and a third vertex group called foo for the remaining vertices.



It is this foo group I wish to decimate:



So far this is good, but if I lower the ratio below 0.3 it goes wonky:



Well, I almost have what I need. If I apply the same decimation again, that should reduce the detail sufficiently.  i.e. 30% * 30% = 9% ~ 0.1 which is what I was after.  But applying it twice, the second application gives another wonky picture the same as the one I posted. 

Also, I shouldn't have to be sidestepping around weird behaviour I don't understand. Is it a bug, or am I doing something wrong?

PS I guess I can manually create a new Vertex group and decimate that...
The ratio on the decimate modifier doesn't refer to level detail, it refers to the number of faces. From the wiki:


  Ratio: The ratio of faces to keep after decimation, from 0.0 (0%, all faces have been completely removed) to 1.0 (100%, mesh is completely
  intact, except quads have been triangulated). 
      As the percentage drops from 1.0 to 0.0, the mesh becomes more and more decimated until it no longer visually looks like the original
  mesh.