It's just what I heard from some other students. I think it was based on a combination of factors. Namely, that they're usually NOT the same thing. Summer internships are usually full-time and gained through the general hiring process. Externships are usually done during the school year, part-time, usually formed through the school's contacts or an established program, and are done for coursework. Externships may be seen as requiring less commitment from both the employer and the student than internships.
If they're both full-time summer gigs and one is just called an internship and the other is just called an externship, then there's obviously no difference. But if the general distinguishing factors above are true, that may support a slight prestige bump for an internship.