I believe it was our good @CapnZapp who first postulated the lazy fey idea.My favorite explanation for Barkskin was lazy fey spirits. They only work as hard as they need to reach 16AC, then slack off.
I don't remember who first came up with the idea, but it's stuck with me ever since.
I actually find it an amusing character idea. You're a character that has a swarm of fey spirits all around you all the time. When you cast Spirit Guardians you call them to action and they get to fly around attacking everything in sight when you move close to them. They are bloodthirsty little things and love to just kill, kill, kill.
But then you decide to cast Barkskin and suddenly they no longer get to go around killing things, instead they are stuck going on the defensive and have to block attacks made against you. Talk about boring! Then to make matters worse... if you then pick up and use a shield or move behind cover, they are all like "What the hell is this crap?!? What, the job we doing no good enough for you?!? Screw you, dude!" and they slack off their duties and that's why your AC remains only at 16 despite the shield and/or cover bonuses.
But cast Spirit Guardians and they get all happy again! The lesson learned is... don't piss off the fey spirits.