Area measurement of Mask images

[tlynch]

I'm using Versatile Wand to detect between grass (green/yellow) and ground (black/brown/grey) in photos. I then transform the image into a B&W Mask image (grass=white, ground=black). I can do this OK, and then measure the area %.

However, when I later open that same saved Mask image, and measure the area again, the grass area % is much smaller. Why is this happening, and which measurement is more accurate?

Thanks for any replies.