Very shortly this is how it works. Every day it samples the leaderboard and stores that value. It compares it against the last value gathered and reports the difference.
This works perfectly when the leaderboard updates every day... which it did for a few days. But when it doesn't update (taken offline by turbine... it's just screwed.. etc) then when it does update, it compares the new value to the one last stored, which could be a couple weeks previous (we had a huge gap because the leaderboards were taken down for the EU migration for about 2.5 weeks).