Few reasons why  surnames, birth dates
are non-consistent even in my family:

Child birth registration is/was not compulsory

Parents even did not remember the exact birth dates of
our large family, when the district authority appeared randomly for record
(I remember 2032-2034 BS, my mother was giving approximate birthdates
of our seven brothers and two sisters in my village)

In our family we have three types of surnames (Thar, Gotra and common like Sharma) and few not real birth dates, sometimes it's a matter of headache to get consistency in certificates, old birth records,
district citizenship card and thus passport.

I think it's a hidden mystery for most of the families in Nepal ?!