Renault was granted permission specifically to increase their power and was not done under a reliability guise. This is because FIA wanted to equalise engine performance. However as we all know pure bhp is only 1 parameter, and what Mercedes did not do is argue back to change improve their engine from an fuel efficiency perspective. This is why Renault engined cars can be packaged so much better because they got free power upgrades at a later point in the timeline which gave them enough time to increase performance for free efficiently without affecting other engine parameters significantly.
Merc got more power using more fuel.
Renault was low on power and used less fuel but gained the knowledge and FIA approval to increase power without affecting fuel efficiency. It wasn't fair really. Renault was artificially allowed to play catch up and Merc/Ferrari held back.
I do enjoy bringing this point up as often as possible when in conversation with RBR and Lotus fans. You just have to looooove seeing them squirm, but this OT ofcourse. And I've quit this habit lately as anything can happen in 2014 and then the boot may be on the other foot.
Still, it was fun while it lasted.
(Sorry for the - slightly dastardly - OT mods)