So confused...

I've been trying to figure out what combination of professions will give me best defences(melee and ranged). After trying out a few and have had some ideas what I want, I suddenly discovered that the defence rating also have to do with the weapon equipped as well! WTF. I had melee defence of 203 with Heavy Particle Beam Cannon equipped, but when I unequipped, my defence suddenly became 258. Gee, great, what a cock up ...