It's just how it works. Even my transactions are also send via some "proxy" address. It looks like many input/output transactions are connected and being sent together as one bigger transaction. I can give you many examples.
Here I've paid 0.3 BTC. There're many input addresses and none of them is mine. Total sum is also different:
https://blockchain.info/pl/tx/fc2caa1fcd7bc14ad686635e2ecc7fc38afe9f3692cb608995aa4f9f35461336
Another one - I've paid 0.1 BTC, but tx amount is bigger and sent from another address - but you still can see separated amounts (0.1 + 0.2599):
https://blockchain.info/pl/tx/adf9d028a69c2de8cba76d8cc869023930bb981dc143279b6f118dc5b4454c6d