I don't know the reason but this is what happens:
if there is the article the possessive adjective comes first, if there isn't casa comes first.
this happens with few words and the meaning has an undertone: in Italian the last ward is usually the most important.
- "quella è la sua roba" = "that is stuff of him": meaning that is stuff and it's his. (answering the question "what is it?")
- "quella è roba sua" = "that's his own staff": that's his (not mine, not yours...) and it's stuff. (answering the question "whose is it?")
this works with the word cose (things) too.
with "casa" the undertone is even more important:
- casa mia = my home, where I live. (sometimes with the meaning of homeland);
- la mia casa = my house, the building I posses.