Yuu having a different biology because they're from another world. Specifically their bones are different from those born in Twisted Wonderland.
I imagine that everyone in Twisted Wonderland has magic in their bodies. I think that living in a world full of magic would result in at least a little magic being absorbed into their bodies. As a result, their bones would thinner and more brittle than Yuu's but magic keeps them from fracturing their bones all the time. This could also apply for literally any of the species in Twisted Wonderland.
And Floyd finds out that Yuu's bones are thicker and stronger while trying to squeeze them. Floyd then proceeds to tell Jade or Azul and it spreads throughout the school. Then everyone's like 'Oh, that's why Yuu got through all those overblots with only minor injuries'.