I think it refers to the cross-section and yes, it probably has been carried over from cylindrical snap peas vs. flat mangetout.
Pencil shaped beans are fatter and tend to snap easier (often in my pocket for example) whereas the flat types - though just as crisp - tend to bend to a C shape before snapping.
Of course you can get pole (french climbing) varieties in both types, just as you can get dwarf frenchies in both types.
The seed catalogues will use whatever name they think will catch your eye better - no matter how misleading. All the way from making stuff look 'NEW' without actually saying so - and up to (sometimes including) actually telling porkies.
However it is definitely worth growing all 4 categories - one will always do best in the next season, and it tends to be a different one each year - you never know which one it will be...
Cheers.