The reason for existance of those variables is that those numbers are variable from computer to computer (depending on the resolution), and variable depending on your monitor setup.
Monitor1, is always the primary monitor. Thus, its Top and Left are always zero.Its Right and Bottom are always equal to Left+Width and Top+Height, respectivelly.
BUT, on monitor 2, things don't work like that. That's because Monitor2 is placed on a relative position to Monitor1.
In your case, with 2 monitors, if you have Monitor2 on the right, the Left will be 1601, or.. Monitor1Top + Monitor1Width + 1. The Top will be the same as Monitor1, 0.
If you have it on the left, its left would be -1601, because it'd be Monitor1Top - Monitor1Width -1. The Top would remain the same.
You can now imagine how things can get messy if you'd have 2 monitors with different resolutions, and not horizontally aligned (like i do
The idea behind having grids defined like this is that you don't have to do this math, GridMove does it for you. Also, if you use those variables, you can share your grids with others, because it isn't hardcoded for your resolution.
PS: to better understand what i mean, please try doing the following procedure:
Go to "Display Properties" -> Settings tab, then, just click and drag one of the monitors. That allows you to get them on the left, on the right, on top, bottom or even "southwest", "southeast", etc