I think I'm going to try using the 940nm ir to see if hunting more brushed in areas is better. There is another post in the optics section where a users experience some good results under these conditions.
I really think that is simply a matter that the 940 isn't as bright or has as much throw as the 850, plus sampling of circumstances. I have used both. Both reflect off of vegetation. How much light is being reflected back will depend on how bright the light is that is being projected, distance to the plants, and reflectivity of the plants. You really need to do a side by side comparing at the same level of brightness, not the same setting of power level because they are not comparable, but same level of brightness.
Photographers have this same issue of flash and foreground and background lighting as well which is not resolve by wavelength.