Why didn't Reese recognize the Terminator?

When Reese and Sarah are peeling all over Los Angeles away from Tech Noir, Reese says that the Terminator is a "Cyberdyne Systems Model 101."

"Model 101" means that its skin covering is the one that looks like Arnold Schwarzenegger. If Reese was familiar enough with the model 101s to recognize the Terminator as one, why does he later tell Sarah that he had to wait before the Terminator moved in on her to be sure it was him?