Different Word Count in Microsoft Word

Can anybody give me an explanation of why my Scrivener document would have a different Word Count than Microsoft Word?

Not sure why the same document will be inconsistent.

It would be surprising if they were the same. That would depend on software engineers in different parts of the globe both deciding that something like “multi-racial” should be counted as one word rather than two, and remembering to tell the software that “e.g.” is not two sentences (and finding a way of doing that while correctly identifying what actually is a sentence). It’s all down to human decisions, and implementing those decisions.