It’s less about “dungeon” v. “cave” and more about functional versus spatial.
Caving allows the character to remember functional routes. The “map in your head” is a step-by-step navigation akin to Google Maps. So it’s helpful in guiding you to a location in functional terms - such as how far and in what direction it took to get to a destination from some other place. It’s not helpful in giving you that place’s spatial relationship to other points.
“Take the large tunnel about 100 paces, then turn leftish at the carved boulder. From there we go about 60 paces, and take the side tunnel to the chimney…”
Mapping allows the character to draw pictorial maps. The ability to read, let alone draw, pictorial maps was a rare skill in Antiquity.
As one source wrote, “maps are a distinct information system from written information. Most people today have little trouble converting locational descriptions into planar systems but this wasn’t necessarily as intuitive for people who rarely used the concept… Ancient travelers used distance and reckoning. They had the ancient equivalent of the steps you get from Google Maps when asking for directions. The Roman road network was a big help in this regard. When at sea, traders had lists of ports, how far away, and what you could expect to find there. However, it was quite rare for someone to have a pictorial representation of this information.”
I hope that helps clarify the difference! You could certainly collapse the two proficiencies into one, or rename “caving” to “underground navigation” if that helps clarify the difference.
Caving (G): The character has learned to keep a map in his head of where he is when exploring underground caves, cavern complexes, and rivers. On a proficiency throw of 11+, the character with this proficiency will be able to automatically know the route he has taken to get where he is, if he was conscious at the time.
Mapping (G): The character can understand and make maps, even if he cannot read or write. With a proficiency throw of 11+, the character can interpret or draft complicated layouts or map an area by memory. This proficiency can be selected multiple times.