Names shown on maps are based on any available sources which can be gathered. The sources often conflict, so a choice needs to be made.
For Ben Vane (Dunbartonshire)OS Name book entry:
https://scotlandsplaces.gov.uk/digital-volumes/ordnance-survey-name-books/dunbartonshire-os-name-books-1860/dunbartonshire-volume-02/31- The consensus appears to have been Ben Vane. Notes exist on the gaelic Beinn Bhàn but English orthography has been used in this case.
- Another question could be: why was English orthography used, not Gaelic? An answer could be that the local people who gave spellings used English orthography.
For Benvane (Perthshire)The OS Name books no longer exist. From
https://scotlandsplaces.gov.uk/digital-volumes/ordnance-survey-name-books/perthshire-os-name-books-1859-1862: "During the Second World War some of the Perthshire OS Name Books were destroyed during an air raid on their storage site at Southampton. Among the volumes destroyed were those covering the whole extent of the county surveyed at the scale of 6 inches to the mile"
So we cannot use the Name books to get an idea of why a name was chosen. But we can look at the names that were chosen, they are on the six-inch map:
https://maps.nls.uk/geo/explore/#zoom=15&lat=56.2925&lon=-4.3695&layers=6&b=1It shows both "Benvane" and "(Beinn Bhàn)". What does that suggest? Perhaps the majority of sources gave the name in English orthography while others used Gaelic orthography.
Looking at the Name books we can see that names are certainly not just the products of cartographers' fervid imaginations. If the name "Benvane" was used it would be because that's how most of the available sources spelled it.
Incidentally, these folks
https://spns.org.uk/ appear to eat sleep and breathe place names and are having a conference in Arrochar in November, pretty well at the foot of Ben Vane.