It can be. Well, it is. Anyway...
Any time the number of rebel controlled provinces in a nation exceeds 'loyalist', the government will fall.
When the government falls, all its wars instantly cease. Any provinces controlled by other nations are immediately ceded to them.
So, for example consider a four province nation that's in your way. You glance over and notice they have a nasty revolt in progress and lose a province. You declare war. At the moment the 'score' is: Them 3, Rebels 1, you 0
Later in the war you've taken your province, but the AI's showing its normal reluctance to listen to your peace offers. Meanwhile the rebels have moved on. (Them 2, Rebels 1, you 1)
The rebels take their second province. (Rebels 2, Them 1, you 1) Their government falls. Your war ends immediately. You get the province you controlled. Note that for the 'government falls' calculation, your province didn't enter into it...it was simply a question of Rebel vs. Loyal provinces. You'd taken that one province out of the calculation.
Obviously for this example it's hardly worth it...but now imagine doing something like this to China in 1645. You take the provinces you want, and just sit there til the rebels bring China crashing down. You can easily get more than 100% worth of provinces that way.
