Goodput Definition

BMWG,

I was asked to review a draft in the AQM working group 
which folks in BMWG may find interesting:
https://tools.ietf.org/html/draft-ietf-aqm-eval-guidelines-00

At the moment, I'm looking at their metric descriptions
for Flow Completion time and Goodput.  These two metrics
have a third factor in their relationship: how much data was sent.

Flow Completion Time = (original data volume, bits) / (Goodput, bits/s)

BMWG has a definition of Goodput in RFC 2647:
http://tools.ietf.org/html/rfc2647#section-3.17

   Definition:
     The number of bits per unit of time forwarded to the correct
     destination interface of the DUT/SUT, minus any bits lost or
     retransmitted.

I understand why retransmitted bits are removed, but bits forwarded
to the "correct destination interface" seem not to be lost.

I wonder if author David Newman or anyone else can recollect why
lost bits must be removed, and where they were thought to be lost?

regards,
Al