You could be on to something. Maybe Siri’s ML model just rounds up or down numbers to the nearest integer, which works fine in most cases but not birthdays.You are much nearer to being 28 years old than 27. To the nearest integer, Siri has correctly rounded up your age.
Anyways, the most annoying thing about Siri is that she doesn’t seem to learn new stuff. I’m in a 15 year long relationships and Siri still asks me which phone number to use for me partner.
It’s the mobile one, for crying out loud.