My flickr friend Jimmythefly drew a set of bikes to show how, if you keep the saddle to bottom bracket (BB) relationship the same, seat tube angles can make bikes with the same dimensions (top tube and seat tube lengths) can fit very differently.
Because of the interrelationship of frame angles influencing tube lengths, two bikes in different “sizes” (seat and top tube lengths) can fit exactly the same.
It’s non-intuitive. It’s weird. It needs a picture to really see how it works. For me, I needed to see the drawings overlayed, which is why I made this GIF. Thanks, Jimmy, for the clear illustration and explanation.