No long term contract has a fixed contract wage over time. It will have suitable improvements over the contract term. Improved contract terms are offered to stop teams poaching and luring players with higher wages.
e.g.
Yr 1 £100K
Yr 2 £115K
Yr 3 £130K + Loyalty Bonus at season end
Yr 4 £150K….
However, once you become integral to the Starting XI, and are getting noticed(!), most agents would start sounding out the Club about a reworking of the terms and the Club would seek increased duration.
e.g. in Palmer’s case, this might be something like…
Yrs 1 & 2 remain as is, to ensure it’s not a one year blip, but maybe a goal/trophy bonus added.
Yr 3 £200K
Yr 4 £225K
Yr 5 £250K + another loyalty bonus
Yr 6 £275K
Yr 7 £300K, plus the addition of 2 yrs
Yr 8 £300K
Yr 9 £300K + loyalty bonus
Yr 10 (Club option) £330K
That is a very simplistic example of how I understand current contracts to be structured. Of course, there are myriad other things thrown in there, including trophies, CL participation, PL wins, Image rights, etc, etc.