Why have I only thwarted 30% of Harley's Revenge?
Solution 1:
I played through the Harley DLC twice an noticed this, at the time I was confused but it climbed up to 100% when I completed it the second time and then to test what I suspected was going on I restarted a 3rd time and the percentage reset again.
So I am pretty sure it displays the percentage of your current saved play through.