I agree.
With PES 2011, you really do have "those days" at either end of the spectrum.
Yesterday, I couldn't hit a barn door, couldn't beat a man, couldn't build up play properly as well as scoring shitloads of own goals and conceding very cheaply (plus the things Jimmy mentions)... Basically everything that could go wrong, did go wrong, I played crap.
One session last week though, everything I touched turned to gold - I was constantly turning defenders inside-out with Nene + R2, playing magical manual passes with Pires, banging in the goals with Keane, whipping in deadly crosses with Kranjcar... Also shutting out everyone, hardly putting a foot wrong in defence.
I even got a win streak of 10, which is my best in MLO so far, and as Jimmy says, I was pretty much "unplayable" - even D1 teams with the likes of Chiellini, Maicon, Ronaldo, Arshavin etc with win percentages well over 60% didn't stand a chance.
It's too easy to blame this on scripting, but as in real football, there are just those games where you're not "in the zone" and nothing comes naturally.
At my best, I don't even need to think about most of the aspects of my play, I just picture something in my mind, and it ends up playing itself out as intended.
On the other hand, when not playing well, every little thing needs a lot of effort and always seems "forced", at these times it's probably best to hang up your boots (or controller
) for the night, it's not often you play yourself out of this rut (however, my spectacular Nene free kick posted in the MLO thread turned a game on it's head and triggered a 3-2 win from 2-0 down

).