It should say 5 known words, but instead one can see that there are 9 ;(
Unfortunately, everyone has this issue and there has never been an official response to it. I wrote a post about it months ago but it got buried with no actual response as to why the issue was happening, sadly.
I know the team is quite busy, however the known words count is quite a significant stat (it is literally on every page you’re on because it’s on the menu bar), so I wish that it worked with more accuracy. Otherwise, I’d simply love to know what the cause is behind the real stat being off like both you and I have shown.
I’ve noticed that sometimes, once I’ve provided definitions for every unknown word in a certain lesson, the unknown word count for that lesson is nevertheless greater than zero. If I then complete the lesson, this excess number of unknown words will be added to my total know word count.
If you simply refrain from completing such lessons, I think you’ll be able to avoid this issue going forward.