On why my Granny did not want a fuss

We told my Granny that we had organised a big family / friends gathering for her birthday. She looked appalled. She told me to cancel it. After half hour of coaxing I found out why she was so utterly mortified. She has basically "under reported" her age. By a few years. And all now know. Poor Granny. She does not look any of her 21 years!