I asked this in the general forum so didnt want to repost, but the general forums are manic at the moment and my thread was off the 2nd page in about 2 minutes without even being viewed.

http://www.swtor.com/community/showthread.php?t=201183

Gist is what is the diff between the armour ratings on light and heavy moddable gear.

Heavy, Medium and Light armor while having the exact same rating will have different armor values. Heavy > Medium > Light.

So if you, as a heavy armor user, obtain a piece of orange light armor and put an Armoring mod in it it will always remain inferior to orange heavy armor with the same Armoring mod.