Probably some source started it by calling it a 20mm weapon and it got copied by others who started referring to each other etc. It's not that strange since the 20mm gun was the smallest AT cannon the germans fielded on tanks and the IC did have an AT weapon. Except that it was more of an antitank rifle type weapon, not a cannon.
Here's a link to a debate on this on another forum:
http://forum.axishistory.com/viewtop...=ew141&start=0
narwan