Why is the Rubisco enzyme more appropriately called

It acts as both carboxylase and oxygenase, so it is referred to as RUBP carboxylase–Oxygenase.

Rubisco binds CO2 to 3PGA in C3 plants. In C3 plants, Rubisco is binding with O2 in a process known photorespiration to generate phosphoglycerate and phosphoglycerate. Photo vacuum reduces carbon attachment in C3 plants.