Not so ASSAM. A flat rise for all eventually puts everyone on the same wage (save for an insignificant amount). A percentage rise retains the same benefits everyone already has.
A rather simplified example.
Person A is paid £12,000, and is able to live a lifestyle of level, say, X
Person B is paid £24,000, and is able to live a lifestyle of level, say, 2X
Over a long period of time inflation takes it's toll and prices rise, say 5000%
Person A is paid and extra £600,000 making £612,000, but because of inflation they can still only live a lifestyle of level X
If Person B is forced to accept the same £600,000 then they get £624,000, and their standard of living has dropped to just 1.02 X. Basically it's fallen through the floor for them. They are now little better off than person A.
But of Person B is given the same percentage rise then they get an extra £1,200,000 making £1,224,000, and taking inflation into account they retain their original lifestyle of level 2X.
The figures are larger for person B, but the value to them is the same.
Differentials are a different subject/argument to pay rises.