I was told the issue lies in the internals of the extension.
Cable to extension may well be capable of 13amps but that some of the internals of some extensions are not all capable of handling 13 amps as they assume the load will be split.
No idea if its true its just what I was told by a sparkie at work
That maybe true of some cheapo gear, if you look inside them it is just normally some flex linking each outlet!