Now though, nothing seems to be able to make the original document catch up, and when I add or delete words it counts upwards or downwards from 6,548. This morning, first thing the document displayed over 7,000, but has now dropped, which is what made me realise it was wrong. I pasted the whole lot into a new document, which then showed 7,045 as the bottom-of-screen count, so that's obviously the correct one.
The bottom-of-screen word count it is currently saying '6,548 words' but when I actually click on the word count the in-window count tells me 7,045. However, my document keeps telling me different things. I love the word count at the bottom of the page, and especially the feature that it tells you how many words you're up to, out of how many in total, so if you have notes after the main body of your work you don't have to keep highlighting the relevant section. Very weird word count issue on my new Macbook Air.