Sig 2022

The name is just marketing. It differs considerably internally as well as externally from the "classic" aluminum-frame SIGs, but I don't know why this would not make it a "true" SIG.

It is because it is actually made by Taurus. Note that Taurus named their Milleniun, during the same period as the Millenium Pro.
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Just kidding....but now the rumor is started. Try to stop it!

I think it just stems from the development - originally as a service weapon with modular components for easy servicing and detail stripping. So SIG Pro for professional use pistol - ie. by professional law enforcement and military units.