The age column on the 1921 census is years and months, not just years. If FMP have transcribed both years and months, I imagine they will have to rejig their normal age calculation, which, as with most genealogy sites, is more often than not a year out since the censuses were taken in the first half of the year when most people would not have had their birthdays yet. It would be a bit more obvious if the year of birth was a year out when the age given is so much more accurate! It might be a bit confusing if the form says April and the census was actually taken in June, though.
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