I don't know why this happens but it happened to me also... only on the birthdays that didn't have a year in them and it defaulted to the year 0001. I changed the year to 2010 and the birthdays instantly showed on the correct day.
I have a similar problem, but only with some of the birthdays. For some reason two appear one day and one year earlier than I have entered them in address book. I would like to know how to sort out the reason for this and correct the errors.