what are Ghost Acres?

The acres the attacker gains but the defender doesn't lose.

Example:
I do a grab for 300a(600a). 300 acres is what they lost. 600 is what I gained. So 300 acres from them and 300 "ghost acres."

Look at Tmac with all the helpful advice ;)

just to be clear since the numbers are similar in Tmac example.

250(850a).

850-250 = ghost acres, which equals 600

=D