You aren't quite comparing like with like. The £108 Off-Peak Return from Burton is routed "via London" and allows travel either via Derby on the MML, or via Tamworth (and optionally via Birmingham) on the WCML. It has no operator restrictions and is thus priced on the assumption that you're going to use 'intercity'/premium services (EMR or Avanti, as well as Gatwick Express on the south end).
The £63 Off-Peak Return you're quoting from Tamworth is restricted to WMT & Southern. It therefore has to be priced cheaper to compete with the equivalent £83 interavailable ("not via Coventry/Birmingham") fare - given the reduced frequency and flexibility you get.
That said, there is certainly an element of Off-Peak walk-up fares being cheaper on the WCML than the ECML, and Burton's tickets being priced higher based on their MML validity.
The difference in fare levels is a result of the WCML twice being upgraded at the 'right' time - it was electrified back in the 1960s, when fares were largely still mileage-based, and the early-2000s Route Modernisation happened during the early privatisation era when fares were roughly tied to inflation.
By contrast the MML was upgraded with HSTs and saw linespeed improvements in the 1980s - at which point market-based pricing was in full force and BR often brought in big fare increases to accompany service improvements. In fact it was usually part of their business case for them. Midland Mainline and EMT did increase their Off-Peak fares quite a lot but they were somewhat constrained by Super Off-Peak fares being regulated fares.
