As the 120 roll has the same surface and I use twice the amount of developer, half of the developing oomph must still be there after the prescribed development time, and the average concentration during development is much higher.
Part of the flaw in your logic comes from believing there is such as thing as "developing oomph" and that it can be precisely divided in half. Film development is a two-way chemical process, It's not only the film that is affected, but also the developer. For example, during the development, oxidation takes place, and the entire developer is affected. Doubling the amount of developer used doesn't change that.
There are other chemical processes that take place, but, also, there are many factors that influence rate of development— and, hence, exhaustion —, such as temperature, dilution, and agitation. Quantity of developing solution is not one of them, in the sense that putting more of the developing solution but with the same dilution, same temperature, same agitation will have little effect on the rate of development of the complete solution (unless, of course, you're talking about much larger differences of ratio in volume than 1 to 2).
Not to mention that rate of development is also affected by the density of the latent image itself. A negative with lots of highlights will not exhaust the developer the same way as a negative will mostly shadows.
Is your complete solution less affected by the various chemical processes at work if you double the volume? Perhaps. By how much? Impossible to tell withouth a whole bunch of highly scientific testing.