If I may put in my personal take on the situation.
In sport, as in many things, it is not necessarily how good you are, but how good you are at your worst. for example, Tennis players like Nadal, Federer and Sampras were able to grind out results even when not fully firing, whereas players like Henman would not all the time, despite being able to spring amazing results, and play exceptionally well in some matches.
I see a similar situation here. When both are at their best (something I am not sure we have really seen, certainly not for an extended run of races) they are close, and are both proven race winners.
However, when not at their best, it is a different story, looking at Canada this year as a prime example.
It is simply the likely range of performances that can be had that makes the difference.
Lewis would always be expected to be fighting out at the sharp end, regardless of how good the car is. Jenson, can, however, less often, due in part to the likelihood of getting the car underneath him.
So, Jenson relies on his ability, and that of the team to get the car sorted, then he can do the rest. Lewis can do the business regardless, with reliance on the team for a car with underlying pace and reliability, but inherently, rather than being significantly faster when both at their best, he is less likely to be significantly slower than his best in other circumstances.
I think that made sense (to me at least)