not a 100% sure but I always assumed att value is their regular attack value and the charge value is a "bonus" that gets added when the cavalry unit sucesfully charges an enemy unit...thats why you see a lot of casualties at the exact moment of the charge and then the death/ratio slows down....In any case with cavalry units you should always try to charge (preferably from behind) retreat them and charge again...rinse, repeat till they break....and then chase em hehe.