I've noticed that too. I think the dominant Ne makes ENTPs appear more talkative than ENTJs who have dominant Te and its the talkativeness that's making ENTPs appear more extraverted. Ne rambles on and on more while Te is more straight to the point, getting rid of extraneous stuff. So ENTJ doesn't appear as talkative.
Also the tertiary functions might have some effect. ENTP has tertiary Fe which is useful for connecting emotionally with other people. ENTJ has tertiary Se which is more about action and let's just do it. So again, ENTP has a tertiary function that makes them appear more sociable, which may make them appear more extraverted.
Of course there are lots of ENTPs who claim to be near the border on E/I and ENTJs who are strong Es. But ENTPs appear more hyper when they are extraverting while ENTJs appear more calm and measured.