Think of it this way, take a regular, legal set of adult pornographic pictures. While legal, we'd still classify this set of pictures as known porn if we were tracking it.

Now the first few images might be the model completely clothed and not even be porn, maybe there's a picture of her lounging around a pool, then another picture of the pool itself. Still its part of a set of pictures that is known porn.

Heck most porn starts off with actors being clothed (so I hear lol).