Assumes (uh oh) that the areas deforested were at or above average density, that no areas became more dense, and that measurement in deforesting areas is as aggressive as in reforesting areas.
That last one is particularly tough, since the forestation counting task draws folks who drink the Kool-aid "straight" and let their faith ("Ice age is coming! No, wait, we're melting! No... wait... freezing!") influence their work.