Session Targets and duplicating docs

In WinScriv if I start Document A, type 10 words then duplicate that document, my session target reads 20 words.

If I then delete 8 of those words in the duplicated document my session target now reads 12, as I would expect - the 10 words from the original document plus the 2 in the new one.

But on MacScriv doing the same thing yields different much results. 10 words into Document A, duplicate, and my session target still reads only 10, delete those 8 words and suddenly I’m back down to 2, rather than the expected 12.

Add a few zeros onto the end of my example numbers above and you’ll see why I was so surprised today. I typed, I don’t know, 3000 (?) words into Document A today. Finished that one and proceeded on with Document B. Document B has the same preamble as Document A so I duplicated Doc A, deleted what I didn’t need and carried on. A bit later I checked my session target and was shocked to see I was only at 121 words. Not believing my eyes, I experimented a bit and am now apparently at -627 words for today when I’ve actually typed lots. (Good thing I didn’t have Allow Negatives checked the entire time, or I’d be somewhere around -9000.)

Is there a setting somewhere I missed? I’d much prefer to have it count the way WinScriv does - I’ve been using session targets to get a sense of my daily output, but if it resets each time I duplicate and delete then I’ll have to come up with another system.