Yep. Frustrating.
Counting words is one of those things that's inherently easy for humans to consider and understand, but difficult for computers to perform accurately in a similar manner. Even Word has a set of rules and gets it "wrong".
1. Hyphenated words sometimes count as two or more.
2. Two words separated by a non-breaking space count as one word.
3. Using a dash with space around it - instead of an em-dash - may count as an extra word.
4. Separators like ~oOo~ are considered a word.
5. They can consider "invisible" web markup as words, so large chunks of whitespace or formatting like bold, italic, etc can bump the word count significantly.
And so on...