You are unerringly accurate.
The bra was a gift from a fan and is the wrong size. The model herself says she is a UK 36L US 36P which was roughly where you were heading until the bras actual size confused things.
What can I say. It’s a sixth sense. “I see boob people.”
Seriously though, with a lifetime of two big boobs hanging from your torso, wearing the wrong size for years, falling in love with proper fitting bras for big boobs, and becoming obsessed with making sure women are wearing the appropriate size - you get a sense for it. I can often just glance at someone and figure out what size they should be. It’s much easier in person - and, oddly, easier when they’re wearing a bra (or sometimes clothed). Weirdly, it’s slightly more difficult to assess a pair of naked boobs (and I’ve seen hundreds of pairs of boobs naked).