Other mistake: At or just after the point where Steve McQueen restrains the diner owner from running out the door - McQueen's wedding ring is visible on his left hand throughout one entire long-shot. (He was 27, despite playing a teenager, and had been married to Neile Adams for more than a year).
Other mistake: In the scene where everyone is running out of the theater most are cracking up. (01:11:00)
Other mistake: The movie supposedly takes place in the Downingtown, Pennsylvania area (although the theater is in nearby Phoenixville). On the (supposedly) Downingtown police station desk, under the phone, is a phone book for the Seaford/Laurel area which is in southern Delaware, almost 120 miles away. (00:43:53)
Answer: Because movies aren't always set in the year they're released. Absent any conflicting information, the assumption must be that the events of the movie take place around July 1957.