For the perk 'Matching Set' does that mean the the matching items must be all of the same kind, ie Thieves Guild or Nightingale eg or just Light Armor vs Heavy Armor? I had saved my game before Leveling Up and noted my pre-armor value and then compared it to my post Level Up, and selecting Matching Set, there did not seem to be any difference. I had been wearing all Light Armor, ie Thieves Guild Hood, TG Boots and TG Gloves but with an Elven Guilded Armor.

Matching Set requires equipments from the same "set". Since your body wears Elven Gilder Armor, it doesn't count

Also, Elven Gilder Armor is in a different set than Elven Armor. Go figure...

Matching set is pretty worthless, as it's far more likely that you'll be wearing mismatched armor as you level, and you'll reach the armor cap by the time you consider top-tier sets.

I think it means all glass or all dwarven ect. All the same part of the smithing tree.

Wow... that's a pretty terrible oversight.

I know, right? The same thing, just more polished, with additional transparent protective layer, and suddenly it's not Elven anymore

IDK what they were thinking. Perhaps they added the equipments automagically with a piece of code that basically says this:

If an item has a name like this: "Elven XXX".
Two Words Only

Then it's an Elven set!

Or they pre-defined the sets, which only had room for one of each "Slot"

Matching set is indeed for MATCHING SETS of armor, oddly enough you tested it with a set that is bugged. Most armors do work properly, however the thieves guild armor does not receive the bonus for some reason. You can confirm it by reading up on either the elder scrolls wiki or UESP

There seems to be three sets of Elven armour: gilded, ordinary, and light. I have no idea why...

Matching set = all the same type of armour, not the same class. For example: All plate armour, all hide armour, all steel armour....not plate cuirass, dwarven boots, steel horned helmet, ebony gauntlets.

To be fair, Gilded Elven is made from different materials, requiring quicksilver, as opposed to the leather of normal Elven armour. Implies it is a heavier beast altogether (yes, I know leather weighs more than an ingot, but thats encumbrance, not just mass.)

Well, since it is named "Elven Gilded Armor" (still bearing "Elven" part), most people will assume that it is from the same set. I thought that Banded Iron is in the same set as Iron, and I think I'm wrong.

Honestly, though, I don't see how real-life logic could apply to the perk in the first place (perhaps you simply got comfortable in it? But it doesn't explain the increase in overall armor rating, which doesn't have anything to do with comfort), so I guess that's Bethesda for you