Oh, so you have run into a problem that's been a real pain in the butt for us as well. Due to the way that stats are aggregated internally, the way that activities have been defined, and the way those definitions have changed since Destiny first launched, stats about Nightfall and Heroic Strikes are intermingled at the deepest level in ways that we cannot untangle. We're planning on fixing it for post-TTK Nightfall/Heroic Strike activities in the near future, but old Nightfall and Heroic Strikes stats (pre-TTK) will forever be, unfortunately, totally -blam!-ed. Because it was -blam!-ed from the start, we haven't made it a priority to fix those activity modes - as you probably see in our UI, we don't let you filter down to Nightfall and Heroic Strike level either because this data is totally borked. It is something that actually recently came to our attention again as we were taking another look at stats, and it is something that we will strive in the near future to at least fix for data post-TTK and onward. We'll keep you all posted here when we un-blam!- that data. The side effect of that fix, when we put it in place, is that very old activities (pre-TTK) will likely misreport as Nightfall when they were, in fact, Heroic Strikes. But it will at least be fixed for data going forward from that point in time when we get to fixing this.