i have no idea, but i'm glad that you (and others) notice it too! totally crazy, and yet demonstrably the case.
my own theory is that it's related to fat intake and healthy oil in the skin... or maybe better vitamin absorption due to the fat. but when i used to do a full-on ketogenic version of paleo, i literally didn't burn and didn't even get uncomfortable. after giving that up and adding in more carbs and a bit less fat, i still don't burn and its tougher to tan, but i'm not quite superman, like in the keto days. honestly, not that i long to be back in ketosis (god no!), but there's something about high fat/ultra low carb that has a profound effect on your ability to burn or tan. weird!
"dean ornish and dr. davis think the palmitic acid our bodies use for fuel while we sleep is poison if we eat it. zero-carbers like charles washington think the oldest fuel in our evolutionary history – glucose - used by organisms a billion years ago and without which the brains of modern mammals cannot survive for more than a few minutes – is an unnatural toxin if you eat it. both views ignore basic facts of medical physiology and defy evolutionary history." - kurt harris