"There isn't some inherent distrustful quality to the being"

Yes, there is. In DnD, if you are a devil, you are lawful evil. Not typically - you are. To be a devil is to be lawful evil - it is not just their nature, but the only way in which they can exist. If you are not lawful evil, or stop being lawful evil, you stop being a devil and become something else.

Devils are also known to be able to disguise themselves as other beings. If they looked like something else and claimed they were better, no sane DnD character with any knowledge of devils would even begin to trust them until they had some kind of proof.

This is indeed projection, but it's actually the opposite of what you are saying. In reality we prescribe the idea that everyone is unique and malleable, that there is no such thing as objective or unsalvagable qualities to a person such as being irredeemably evil. However, DnD is not reality. The rules are different. Devils and evil are an objective combination of qualities that never change, otherwise they cease to be what they are.