“Banjo” is just what instrument 105 is referred to by the General MIDI standard, which is used as a guideline by game developers a lot (e.g. pianos are at the beginning, strings are in the 40/50s, etc), but by no means do they always hold to it (Int Sys in particular barely even had their own standards when it came to this, since almost every song in the vanilla games each has its own redundant pointer table, whose instrument assignments vary). The instrument mapped to 105 by the native instrument map is synthstrings meant for higher notes (which isn’t in either of the “synthstrings” slots, 50 and 51, because those are already taken by samples meant for very low and low notes respectively). Essentially, the instrument list is like a shelf full of CDs where half the cases are empty or have a different CD in them but there’s still kind of a rhyme and reason to it and everyone has their own way of organising theirs.
(I’m pretty sure there’s a banjo in the Advance Wars games if you want to use Sappy to export/import it tho)