why is it different from whitepaper? #132

I am reading the whitepaper on poly official website.

HOWEVER, I find some statements which are different from the implementation in this repo

DIFFERENCE:

• in whitepaper (see following figure), it says that the crosschain-tx is transferred DIRECTLY from source chain to dest chain.
• while in this repo, it says that the crosschain-tx is transferred from source chain to PolyChain and then from PolyChain to dest chain.

QUESTIONS:

• WHY exists this difference ???
• are there any further progress about the statement in whitepaper