I just looked those up, why are you comparing Light armor to Heavy armor? As people have said above the armor rating means different amounts of armor to light, medium, and heavy. Assuming you're a juggernaut from the fact you're looking at heavy strength based armor you should never be wearing light, or medium for that matter.
armor rating is not a definitive number for each piece of gear it's in, it will change based on what type of item you put it in (chest, legs, feet) and what the weight of the armor is (light, medium, heavy).
In short don't expect to put the same armoring mod in a pair of light boots and a heavy chest and expect them to even be close
Edit: Sorry Vanor, missed your post. Had this window open a while