Still don't get the Attack Rolls

all right, this has been bugging me ever since I first played the game 20 years ago, I finished it many times but as much as I hate to admit it I never could figure out what the Attack rolls meant!!

I get that the lower your Thac0 is, the better cause it goes (D20 roll + enemy AC = hit if greater than your Thac0).. Then why is it that, for exemple, I get the most basic creatures hit me EVERY FREAKIN TIME with my healer/off-tank Branwen ??? She has a magical full plate with a shield giving her an AC of -4 and she keeps getting hit like she would be wearing nothing!!

for instance, I played 10 minutes ago and every enemy attack roll got a +3 ???? Like "Attack Roll 11 + 3 = hit" (it was Black Talon elites I assume their Thc0 was around 10) .. so what the hell, shouldnt it read "Attack roll 11 - 4 = miss" ??

thanks