Session target ads words it shouldn't

If I cut a piece of text and paste it back in, this text will be added into the session target. The ‘allow negatives’ is unticked.
When i tick it, this behaviour stops. However, if I type and delete words, both settings count both up and down, ie deleted words do reduce the count on either setting.
The manual has no reference to ‘allow negatives’ so it’s a guess as to what it does

Haven’t tried but I’d hazard a guess that “allow negatives” stops your daily typed words going below zero. So when you cut, say, 100 words, and you haven’t typed anything new yet, your daily progress will still be zero (when it should be -100 words had “allow negatives” been ticked).

When you re-paste, it adds 100 words on.

I’ve fallen foul of this myself after a mass copy/pasting session and it now thinks I typed 30,000 words in a single day.