Speaking with Bridgestone today, they shared the following...
The A40 is roughly an 85/15 tire, just a bit better off-road than the BW501/502 which are 90/10 tires. The better off-road manners comes from the grooves having a hair more room, they flush out easier.
The front tire profile has changed and presents more of a contact patch when leaned over, compared to the BW501.
The tires also have a a different belting system, one used on their sportbike tires, but now evolved enough for heavy adventure bikes.
The rubber compound is also their latest and greatest.