I’ve worked it out! I copied the list of words from Custom.dic to Excel and sorted to find lots of words duplicated 5, 6, 7, 8 times or more, so I filtered to unique records only, and ended up with 9,000-odd words! Silly me, Word 2010 was only tidying up my old, heavily duplicated dictionary. Sorry to have bothered you guys, who kindly give time to help others.