Not clear. It may be that the IR refected from a "white" target may, or may not, be different than that from a green target.
I don't think it was that the grass was green...but that it was grass. Vegetation always reflects lots of even near-IR; notice how it is aways white in IR photographs.

I've also verified this effect by shooting an electrical service box and the parking lot nearby...both of which look identically "bright" and register the same exposure settings on commercial lightmeters and TTL camera meters...but the parking lot was several stops brighter than the electrical box on my homemade meter.

My hand is also apparently "brighter" than an open notebook, even though the white notebook paper is lighter for exposure purposes.