I've got no idea. But it seems clear to me that we're creating less chances this year compared to last. We seem less effective around the box, even when we're controlling the game. And at the other end, we've been less well organised (and we weren't exactly rock solid last year).
I think Yaya's intensity has gone down. He's not making those powerful runs he did last year, and he's not getting into the shooting positions he did last year (or perhaps he's being more closely guarded on the edge of the box).
I'm not seeing as many crosses into the box. Last year, Kolarov and Navas created some great chances with pacey, early crosses into the box.
Our corners are even worse.
But basically, I think it was always a vulnerable system, it's just that last year, we could outscore anyone in the league. This season, our forward play is nowhere near as good (with the odd exception), and defensively, we're just as vulnerable.