As far as I can work out that's a 2007 Rocket 105 on the left and a 2008 Rocket 115 on the right. I'm guessing in 2007 The folks at Tabou thought "We've got to do something to make them ride a bit easier" But I'm thinking that, as well as concaves, weight, stiffness and centre of percussion also have something to do with it.
en.wikipedia.org/wiki/Center_of_percussionAs you get older these things become more important. I'm thinking of dusting off the old Bombora 270 I've got under the veranda and taking it for another spin.