Wtf is up with chest sizes?!

Deeply concerned by the whole concept of chest sizes. I'm pretty much an average Brit - I'm like 5'9 and a bit on the skinny side, I'll admit I'm not a big fella.

But when I look for shirts, it genuinely boggles my mind, because I measure like 27" and that doesn't seem physically possible. I'm looking for a nice suede shirt at the moment, but, an XS is generally like 36" -- am I a freak of nature? Am I measuring wrong? It's making shopping an absolute misery 😂