I find really big tops and roots along bluffs (very steep rocky hillsides) here in my area too... and even though the top may be 2-3x as large as my average woods seng... the necks are not all that long. Sometimes only showing 5-8 flats on the neck but being huge 4 prongers.
I think in those ideal locations, it can just get extra large in a shorter time than usual.
The cultivated ginseng that is grown in artificial shade... it just 4-6 years those things are monster 4 prongers.
I think the wild stuff can do very similar growth wise (not hardly that fast, but not far behind either) if it is just in an ideal location.
I have never found a 5 prong myself... but when I do (still got my hopes up).. I would bet it would be on a steep rocky hillside, like along a river or creek bluff.
Congrats on a good year senging this year... and a great way to end the season with that special 5 prong top.
I would also suggest pressing it, and framing it.
Memories made for sure.
TNhunter