john wrote:
 But it is 
 non-directional, so where does the extra gain come from?
You seem to be confusing "non-directional" and "omni-
directional". A non-directional antenna has no gain.
An omni-directional antenna takes away from higher
elevation radiation and focuses that radiation
at lower elevation radiation angles.
-- 
73, Cecil  
http://www.w5dxp.com