There are a few theories why this could be :
Specific genetic anosmia : This is where the person is thought to genetically be incapable of smelling certain odors (or be far less sensitive to the odors). Personally I don't think this is the case in 'fecal body odor', because I can smell feces. I just can't usually smell it when it is from my circulation.
Desensitized to the smell : The theory is the brain (or olfactory system ?) get used to a smell and therefore ignore it. It's possible but again I do not think this explains it.
People and carriers with the same genetic fault have lower sensitivity because the compounds are circulating and reducing the sensitivity of the olfactory system : This is the theory I think most likely correct. My guess is that humans are designed not to smell things that circulate in them normally. Perhaps due to an olfactory reason, or perhaps the brain ignores them. All humans smell to dogs, but we cannot smell most of the smells. However when someone has a genetic fault causing abnormal compounds to circulate, my guess is that normal people will at times detect it, whereas carriers and sufferers will not. Presumably even carriers will notice it when it passes a certain threshold. So when your parents or siblings say they cannot smell you, I believe them. Since I reckon perhaps 10-15% of the population may be carriers too (perhaps more), I'm guessing that potential partners are most likely to be in this group too (?)
On the upside it should mean that most 'sufferers' are 'odor free' among other sufferers and perhaps carriers too. Although perhaps this depends how severe the genetic mutation is. It seems to be a threshold/sensitivity issue, so perhaps severe cases could be smelt by mild cases.
I also think that if it is a different enzyme at fault, then a person will be able to smell another person. Or even ordinary body odor/halitosis, where again 'loved ones' and the sufferer cannot seem to smell the odor