The thirteenth-century writings of Snorri Sturluson are remarkable, but they must be placed in context. He wrote long after conversion, and his work was likely affected by the fogging of the lens caused by time and the shift in beliefs caused by the arrival of Christianity. In addition, Snorri's Iceland was far removed from Scandinavia, so anything that one might say about Iceland might not apply to Norway, Denmark, or Sweden, regardless of the period.
Classifying supernatural beings is always problematic since it depends on the fluid testimonies of informants who might not agree with one another and whose observations can change from time to time. Since people might disagree with the use terms and designations, it is absurd to attach too much importance to these sorts of classifications if one is attempting to understand the folk's belief system.
Regardless of how Snorri intended his terms of various supernatural beings, his specific categories would probably have meant very little to the folk themselves who would have likely argued with one another about the merits of Snorri's work. We encounter problems with attempts at rigid taxonomy organized by the early folklorists of the nineteenth century. Although their work is of interest and is always a starting point, we generally move past their attempts at classification with an understanding that the folk did not always "follow the rules" established by the early scholars who attempted to understand folk traditions.
In short, I suggest you not over think what Snorri has to say about these categories.