The count will match 99.9 percent of the time, but I’ve found an exception. When I use the word 24/7, Microsoft counts that as one word. Scrivener thinks it’s two words. I think it really is just one word, same as a hyphenated word.
Yes, there are slight differences in word count algorithms between applications and between platforms. Once, to resolve a dispute with an editor, I checked a manuscript with three different word processors and got three different results.
If you need an exact count, the best answer is to find out what tool your editor uses, and use the same one.
During Beta, Scrivener fixed a number of word-count differences with Word. Just thought we might want to fix the last one I can find.