Ugh. They are not effectively the same thing.
Counting actual battles instead of the percent increase means that a minor percentage increase in play of the group that's normally played the most could easily surpass a very large percentage increase in the group that's normally played the least.
Example (numbers made up for the sake of the example):
Experimentals are normally played 1000 times per week. Cold War NATO are normally played 100 times per week.
A meager 15% increase in Experimental play would result in an extra 150 battles. But a 100% increase in Cold War NATO play would only result in an extra 100 battles.
The latter is far more significant because the event actually got people to go out and play a relatively unpopular group of vehicles.