Doesn't really explain anything. The total amount of readers increased, but there's no one new in "recent readers". If it was someone old, it shouldn't have increased, and if it was new, they should be in recent readers.
You are way too worried about this; random people will drop your story all the time, you can't please everyone, so at least please those who stayed. Just continue writing as you always had and don't overthink about stuff like this. Otherwise, you'll end up like me with my 7 months of hiatus.
If you need an explanation, I'll try my best to convey how the library/reading list works.
As I said before, just because someone put your story on their dropped list, they won't always appear in "recent readers". The reason being, they can just put it on the list without ever opening a single chapter. And "recent readers" only count people who open a chapter, not your story page.
They only need to press this button on your front page and add it to their "dropped" library without ever pressing any of your chapter links. This also applies to other library tags.
But, that doesn't mean they don't read your story. Someone probably read your story a long time ago and decided to drop it after not reading it for a while (meaning they could be buried under new recent readers or could still be on the list.)