It's all about how your body reacts to it. Apart from vomiting, your body doesnt really do anything about N, but it has specific enzymes to counter what SN causes upon ingestion, albeit they are never used on such a high "scale". That's why I think the recommended dosage keeps rising, as more and more people keep chosing this method, their bodies react differently and 15g might not even be enough to overcome this protection your own body has against methemoglobenia.
If it reassures you somewhat, a user here recently went through SN with 30g and hasn't been heard of ever since.