-4 characters learned today, too many mistakes?

I just checked my stats today to see that the number of character I know went down, is a character or word considered forgotten after too many mistakes?

A word is considered learned if one can remember it for a period of more than 24 hours. So, probably it’s because the scheduled next review has dropped below 24 hours.

if so, what would have caused this?

Each time when the “1 forgot” button is pressed, the scheduled next review will drop by a certain percent.

OK I see, thank you!