Maximum current on 6 AWG wire
The long term current specification for conductors is limited by the temperature rise, and its effect on the insulation. You would expect therefore wire insulated with a higher temperature rated insulation to be able to carry more current.

However, a key word is 'rated', what are the rated conditions? A cable will have a much higher current rating if it's to be used as a single wire in the open air, than if it's to be used with a bunch of similar wires in a closed conduit, because of the radically different cooling performance of the environment round them.

Dig into the conditions under which the two cables are rated. If you can't find those conditions spelled out anywhere, then you may assume that they've probably used a more 'favourable' environment than you would expect. Some unscrupulous suppliers may even rate wire for 'short term use', for instance 30 seconds in every 10 minutes would allow you to show a far higher current capability than continuous.