SCIENCE LESSON WARNING!!
hehe
I'm no doc, but have a very strong background in biology. Having a slower metabolism actually can make sense from a evolutionary biology point of view (if you buy that sort of thing). Means you store fat easier and go through your stores more slowly. Those of us cursed with this would have been in good shape in caveman times b/c we could get through lean times better w/o starving.
EVERYthing in biology is on bell curve (hair color, skin color, height, etc). Having variations in BBT is normal, too. I would be shocked if our low BBT is anything to worry over... but ask your doc next time you are there, just in case!