Progress reduced throughout time for my Math courses

I've haven't been in Khan Academy for a few years, but when I came back I noticed that my progress completed for Math courses has reduced from 100% to smaller percents... Any idea why is this? I can't figure out if content has been added, or I speculate that maybe after time the percent complete is slowly "being forgotten" to stimulate people to keeps studying... For example before my Math by grade said:

6th 100%

Now it says:

6th grade: 68%

Thanks in advance.