I've noticed noticed this for a long time but decided to take a couple of screenshots to show it.
ISP: Telecom Retail
Phone: Galaxy S3 connected to router by 802.11g
Client: Official Speedtest.net android app
Screenshot of Speedtest.net app... these results are repeatable.
Screenshot of the ADSL stats for the router at the same time.
Now I think I know roughly how this happens, but to me (and anyone concerned as to how speedtest.net can report a physically impossible upload speed) this means I can not treat any of their reported results with any credibility.
ps. Oh, the browser app to the same server (fast PC, connected by nice metally copper to the same router) gave more realistic 15.04/0.53 results.