setdiff bizarre

[...]

I suspect it's using 'deparse()' to get the character representation.
This function is mentioned in ?as.character, but as.character.default
disappears into the infernal .Internal and I don't have time to chase
source code - it's sunny outside!
  

on the side, as.character triggers do_ascharacter, which in turn calls
DispatchOrEval, a function with the following beautiful comment:

"To call this an ugly hack would be to insult all existing ugly hacks at
large in the world."

a fortune?

vQ