It is not possible to compare the two bridges as they have different characteristics which need to be considered when setting the toll levels.
The Severn is run by a private sector consortium and the legislation seeks to apply a clear structure to give the concessionaire confidence that it would be able to meet its liabilities in relation to the large risk it took on board. The legislation requires an increase in the tolls each year calculated in line with the retail prices index (RPI). The Secretary of State does not have the power to set an increase at a different rate without the agreement of the concessionaire.
The Humber Bridge is run by a company made up of the four relevant local authorities who borrowed significant public money to enable the bridge to be built. The legislation is concerned with ensuring the Bridge Board meets its obligations for managing the bridge and repaying the debt, but because of the different risks does not restrict the Secretary of State to annual RPI increases.