I have actually had kind of similar issue. Hardly issue, but you know what i mean.
My net is a 1.5Mb/s plan, so i DL at a max of about 160kb/s constant. But one time i got CAPPED , i repeat CAPPED, so my DL speed was down to about 14kb/s but there was a new Portal or TF2 update (can't remember which) of about 4Gb. I started the download nevertheless, expecting it would be maybe 1 or 2 percent when i woke up 4 hours later, but i look and it's 100%, reporting that i had a max DL speed of 1.8Mb/s or something, i was like :O and it couldn't have been a mistake because it had downloaded 4Gb-ish in under 4 hours, despite me apparently being 'capped'.
Tried speedtest.net and normal HTTP downloads and they maxed at 14-15kb/s :S