And for that I am sorry, because I think I finally figured out the difference. A few months ago, I finally found a navy one on sale enough for me to buy it. And I was shocked as hell that it was awful in the heat. So so terrible. It has been great this winter and I frequently wear it. But when it is hot nope nope nope. Do NOT go there.
Of course me being me needed a reason for why they were so different. I started with the fabric content and sure enough all of my other ones are 88%/12% nylon to spandex, but the hotter shirt is 92/8. I don't know if anyone else cares, but it made me happy to finally figure out the mystery of the OMG hate vs OMG love reviews.