It is a coincidence. You have a 1-in-10000 chance of getting any 4 digit number and they assign 5.5M a year, so we can expect that 550 people get their mother's year of birth every year. You just happened to get 1961.
(Total guess but how cool would it be if I was right?)
I have a REALLY hard time believing that but I've never looked into it. Like you said, 550 people a year get it. I just happened to be in the 0.01%? I should be luckier, lol.
