Mysql - MyISAM Default order
There is no "default" order in the RDBMS. Just because tables are not lists but sets. Imagine records as a cards in a big bag, not in a stack. There is no guarantee that cards will be fetched from the bag in some specific order.

Yes, cards are enumerated by primary key, but you have to request the fetching in that specific order. It is not the default order when order is not specified.