My son who is 21 month old has a best girlfriend who was born the same day and year (40 minutes apart) as he is!
This explains it:
In probability theory, the birthday paradox states that given a group of 23 (or more) randomly chosen people, the probability is more than 50% that at least two of them will have the same birthday. For 60 or more people, the probability is greater than 99%, although it cannot actually be 100% unless there are at least 366 people. This is not a paradox in the sense of leading to a logical contradiction; it is described as a paradox because mathematical truth contradicts naive intuition: most people estimate that the chance is much lower than 50%. Calculating this probability (and related ones) is the birthday problem. The mathematics behind it has been used to devise a well-known cryptographic attack named the birthday attack.