WordPress removes HTML entities, convert connectors (– or em-dash) to spaces, removes unwated characters (General Punctuation, Superscripts and Subscripts, Currency Symbols, Combining Diacritical Marks for Symbols, Letterlike Symbols, Number Forms, Arrows, Mathematical Operators, Miscellaneous Technical, Control Pictures, Optical Character Recognition, Enclosed Alphanumerics, Box Drawing, Block Elements, Geometric Shapes, Miscellaneous Symbols, Dingbats, Miscellaneous Mathematical Symbols-A, Supplemental Arrows-A, Braille Patterns, Supplemental Arrows-B, Miscellaneous Mathematical Symbols-B, Supplemental Mathematical Operators, Miscellaneous Symbols and Arrows), removes UTF-16 surrogate points.
You can see js code in wp-admin/js/word-count.js
On my test site, even for quite complicated generated test posts word count is the same. For example, http://test.kagg.eu/at-commodi-quia-enim/ shows 622 as in WordPress, so in Word.
Question what counter is more accurate has not too much sense, as different software defines term “word” on its own.