I’ve no idea why. The text is marked as English language, but the spellchecker doesn’t recognise any hyphenated words unless that specific pairing is also in the dictionary. So it recognises ‘nine’ and ‘inch’, but underlines ‘nine-inch’ as an unrecognised word.

It’s a real problem for me. There’s the added problem that dynamic spell-checking underlines words in the custom dictionary, so you can’t spot errors without running the spellchecker, in which case you spend ages telling it to ignore uncapitalised words in tables and email addresses, abbreviations in tables, etc. The result is that, if I’m pushed for time, I have to ignore the spellchecker and proofread my document the old-fashioned way, checking every spelling with the naked eye.