This is an interesting question and I recommend that you discuss it with your doctor
or your son's pediatrician
. While it is possible that this is just a different manifestation of his genetic makeup, it is also possible that there could be something more at play. There are many rare and unusual conditions that could potentially lead to this sort of finding, but it is hard to know if this is something serious or something normal without more information. An examination of what you are describing would help, but it may be that some testing would be valuable as well. If he is growing well, and meeting all of his other targets as far as development goes, it is possible that this is just something normal that is having an unusual presentation. Speaking with your doctor or your son's pediatrician is the best place to start, so please speak with your doctor.