I don't understand my NM(db) readings
 

I am looking for ways to get my sometimes spotty channels to a more solid strength.
When I run TVFOOL with an antenna height of 20' I get a reading of -12.4NM for one problem channel 2.1
When I increase the antenna height to 100' the reading gets worse at -13.7NM
When I decrease the antenna height to 5' the reading gets better at -9.0NM
This was the opposite of what I expected.
Is there a logical explanation of this?
My TVFOOL is: http://www.tvfool.com/?option=com_wr...5b940158944aea

Thanks for your help
Judd