If 23 people get together, it's odds-on that some two share the same birthday. This is not intuitive, so it's a surprise.
One of the OU modules I work on has about 1200 students. With that number of people, it's odds-on that 10 of them have the same birthday.
Who would have guessed that? If we checked through all their birthdays (probably we can't because of Data Protection) and compared them, then found 10 the same, it might be regarded as an amazing coincidence. But it's not.
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I wrote a short program to simulate this and it supports the theoretical result. With 1200 people it is more than odds-on that some 10 share the same birthday.