It has to do, with when consensus is calculated vs when you get to see your percentage. You see your percentage before the consensus updates. So often times the percentage is in error.
The other part of that, is that consensus is only based on segments a player adds or takes away, the seed does not count.
What does it say, after you refresh the site, for that cube? Because there's also the fact that activity tracker is not real time, so it shows you this, which as atani explained is before the spawner updates the consensus with your validation, and then after a few seconds the spawner updates it, but the activity tracker needs a refresh to show the updated accuracy. So if you still have that cube in the activity tracker (try adding more cubes for a bit from gear cog) what does it say now for that cube?