Recent South East area Franchise Agreements include a formula to share revenue risk based on Central London Employment stats, presumably the idea being that taxpayer via DfT and the Franchisee should not take all the risk/benefit of changes to employment levels, while franchisee still exposed to reduction in usage cus of poor service etc.
GA and South Western have complained that the formula is not working as expected, see eg notes to GAs most recently published accounts. Recent experience seems to show that continuing increases in the Central London Employment stats have not resulted in similar increases in commuting levels. No one really knows why - could be that the CLE stats are wrong or at least dont properly measure the type of jobs that people commute medium/long distance to, or could be that shift to working from home etc has resulted in reduction in season tic purchases indicating a lower level of commuting where actually people have just stopped buying so many season tics. Will be interesting to see how this plays out in the new Franchise Agreement - both Abellio and First been burnt recently, and Govia not exposed to CLE stats under Thameslink concession.