Mumford has never headlined a major US festival, while Jack White has numerous ones. How would that work?
It's not the sole metric by which to work things out but
First week sales of Babel : 600,000 First week sales of Blunderbuss: 138,000
While record sales certainly aren't everything, in this case I think that they're a indicator of momentum. Jack White has been building his fanbase over a longer period of time while Mumford has kind of been shot out of a cannon. That may be what propels them ahead of White on a theoretical line up containing both of them.