It is a fairly well known fact that in any group of people, the odds of two of those people sharing the same birthday is a lot higher than you would expect.  Once the group size is over 60 or so it is practically guaranteed that some of them share birthdays.  The key to understand it: think of how many possible pairs of people there are in the group, rather than just the number of people.  Wikipedia has a good write-up of the math.  If you don't believe it, you can see the effect graphically.

This is a case where binaversaries can act a little differently to regular birthdays.  The chances of people within a group sharing the same binaversary depends on the age distribution in that group; if they are all the same age then the situation is the same as for birthdays, otherwise the odds decrease according to how wide the distribution is.