The answer there is level/competence-based, rather than years-of-experience based compensation structures. If you're only 50% more valuable than someone else, you should make ~50% more, not 100% more. Note that these figures are in total-cost-to-company terms, which might mean your own gross pay would be more than 50% higher if social and per-employee-overhead charges cap out.

Part of the issue is people with 25 years of experience think they deserve more than someone with "only" 15 years and when they don't get it, it's because of ageism or age-discrimination not accurate assessment of value.