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.