I expect I'm wrong so don't flame me - but here me out anyway.
With the news that Lewis' engine has been passed by the FIA, it would be safe to assume that all the teams knew before the German race that the winner's engine eould be the first one to be tested. Yes?
Up to this point in the season, the FIA had NOT been testing engines, correct? And the teams knew they wouldn't get tested. If this isn't correct my theory holds no water.
Theory. Up until the German race (or whichever race saw the introduction of the engine at risk of being tested) Ferrari's engine was NOT legitimate and they had used the time when the FIA couldn't check them to seek out an advantage. Now they have had to resort to the control engine and have found their pace to be lacking as McLaren have made real strides in the aero dept.
Although I doubt this is true...I wouldn't put it passed Ferrari who are past masters at the art of getting round the rules.