While going through record of birth, the year do vary alot because the general public did not know there birthdays.
There can be a million reasons.
A friend of mine from Ireland has lived in the UK since before you needed a passport to visit. She has been trying for nearly 2 years to find her birth cert so she can apply for a passport. I said I would help, I have found out she was born 26th December so was not registered until the following year. Her mothers name was TATTEN but was spelt TALTEN, they had not crossed both T's, her name was registered the wrong way round with her first name as her second name, she has grown up as Sarah Gail but on cert is Gail Sarah. This is way her family have struggled to find the cert. I am going to Dublin next week and will get a copy for her so she can apply for the correct Birth Cert. Ha ha we do not know what to do after, change her bank account change her driving licence. I laughed to her husband the marriage does not count. Wrong name.