Because it's harder to compress the bottom part of the jar. The top part is open so it can't resist the compression. Who can't break an egg in their hand?This would explain his blasé reaction. There is so much about this that doesn't make sense (Why would it break after passing the spincter? What about the compressive strength of glass? Yadda... Yadda...). If people can't even break an egg in their hand, I can assume that his rectum could crush granite.
I don't think his blasé reaction was unusual. I think he realized that if he panicked it would make it worse. And it probably hurt to move once it broke. Seemed like he was trying to stay relaxed because if he tensed up it would cut him more. I would probably react the same way.