Well, because we are comparing different entries with one another. You're right that 2 platforms vs. 1 is not an equal playing field, but if you wait a year or 1.5 years, we can do the same thing without combining the two versions!
There always seems to be a strange reluctance to group multiple versions of Nintendo games together, even if they're essentially the same game or released relatively close to each other. Maybe it's because Nintendo themselves count them seperately or perhaps because Nintendo software has set absurd standards as exclusive, mostly one-off SKUs.
But you often see games like GTAV, FF7, TLOU, Skyrim and various Capcom franchises like Resident Evil or Monster Hunter counted as one game, even though the differences between the versions and the timespan of the releases is often much bigger.
Obviously, it's not really important. But on a List Wars basis, it makes just as much sense to group the aforementioned together as it does for MK8, Tropical Freeze, Pikmin 3 and even the third Platinum/Ultra Sun/Moon versions of Pokemon games.
