The reason the shirt thinks it is rubber is that prior versions of Poser Bullet Physics does not allow objects inside each other.  There is a demonstration of Poser 14 where a bunch of balls fall into a bowl.  In prior versions those balls would fall and stop at the top of the bowl like they were sitting on an invisible top.  Poser 14 now allows objects insid each other which is what makes Poser Cloth possible.

Poser 5, 6, 7, 8, Poser Pro 9 (2012), 10 (2014), 11, 12, 13, 14