In actuality, green grass is an approximation simply because you have variables such as surface reflectivity of the blades of grass, whether or not the grass is damp and the wet surface reflectles even more light, etc.!
But metering green grass is far far better than filling your viewfinder full of black-jerseyed football players vs. full of white-jerseyed football players!!!
Even the widely flaunted 18% gray card is often flawed by use, where the user has the surface reflectivity of the card causing the meter to see more light from the sheen, than if the card was at a slightly different angle and showing the meter only diffuse reading.
Yep, you are right. The reflectivity always drives me crazy when i try to do something "exact" with exposure. Even when i "calibrated" my palm i realized that the exposure changes +/- 1 stop just by changing the angle of the hand towards the sun. I guess this is just one of those things that can never be exact science. But i won't be giving up 
It's just funny how even in the basic books about exposure it's often said that metering is not that difficult and that you can use these rules and get your exposure always spot on. I find a little harder than that..

..Some times though, what looks good on my display screen USUALLY doesn't look good on the pc...


