Mercedes were sandbagging, pretty confident of their advantage after pre-season testing. They let Ferrari get a bit too close for comfort a couple times and it bit them. After that, they still didn't push 100%, but they gve themselves a safer margin.
Honestly, development between them wasn't really any different. As it is for most teams every year. Differences usually come down to how the car matches the tires/track/condition of the given weekend.
Mercedes weakness since taking over from BrawnGP has always been their tire usage. Maybe that can be traced back to Ross Brawn though because even when Ferrari had tailor made tires by Bridgestone it was their weakness. Cars designed by him were great, but not kind to the tires.
IMO with the new engine formula Mercedes was way ahead of everyone, and so the little blips they had with tires were of no big concern. How that develops remains to be seen. Right now everybody is nursing their tires in the race, so other teams don't need to get on the same level with their engines. And the Mercedes isn't the best chassis IMO, that honour would go to Red Bull. Ferrari with a Red Bull chassis, would have run Mercedes closer. So if there is still tire nursing, Mercedes can't simply turn up the engine and be ahead of everyone. Should Mercedes however improve their tire management significantly I excpect them to have a big enough gap for this season.
In any case Ferrari needs to improve everywhere, except maybe in its tire usage. Although it's fashionable to blast Kimi these days, he IMO is still one of the best with tire management and he has shown that last year on several occasions, despite all the other things that were not as stellar as hoped for.
Edited by HP, 10 January 2016 - 19:57.