[Exim] Curiosity: throughput of Pop vs SMTP

This question arises out of curiosity, rather than any burning need.
But if anyone can tell me what's going on, I'd love to know.

I use exim on a Debian system with dial-up (56k) to an ISP to get
mail. When I do this I've noticed (using statnet) that my throughput
on POP3 (fetchmail) is many times slower (1/4) than on SMTP.

It is no surprise that local SMTP is faster than POP. What is
surprising to me is that the sustained rate seems to be higher. I
would think that the fast local connection would drain the pipe and
then sit around waiting for more. But the message which is getting
passed around is not changing size much, so the throughput on each
channel should be the same.

Or is SMTP being used multiple times, something like
POP -> fetchmail -> SMTP -> exim -> SMTP for internal processing ?