In Google Analytics Demographics, why do the male and female user figures not add up to equal 100% (the total number of users within the period)?
There are two reasons why your demographics reports will never reach 100%:
1) A user blocks third-party cookies and does not allow Google to form a profile on them (which means Google can't infer gender).
2) A user is not positively identified as a male/female, age group, etc. because their browsing history is inconclusive.
Usually, for about 50% of sessions, you will see gender in reports, +-10%. I've rarely seen a match rate higher than 60%.
Learn more about Demographics and Benchmarking in the Google Analytics Mastery Course lesson Understanding Your Users.