Hey guys, yet again i need some help decifering tests!

I have just received my BHCG levels ? but I am not going back to see the dr until the 14th of jan, so need some help to answer a question!

Ok so I had the BHCG test yesterday. Day 21 of my cycle. I got the results which were .5HCG. Which doesn?t shock me because I O late in my cycle so even if I am UTD I would be very surprised if it came back positive. So I know that they don?t say your pregnant unless your levels are 5 or higher. But I am confused. I would assume that if you weren?t pregnant your body wouldn?t produce any HCG at all? So what does it mean if your levels are between .5 ? 5??