Are all subjects the same?

University rankers seem to be moving towards the field  normalization of citations data. In 2010 Times Higher Education and Thomson Reuters started using it for their world rankings. The scores for citations did not reflect the absolute number of citations or even citations per paper or per faculty but citations per paper in relation to the world average for 250 fields. Normalisation by year of citation was added to the process. I have heard that QS is considering normalization by five subject groups. Meanwhile THE has switched to Scopus as a data source and they apparently have 300 fields.

This is justified by the claim that it is unfair that an outstanding paper in history or philosophy should be given the same value as a mediocre one in medicine or physics, something that could happen if only the number of citations were counted.  Perhaps, but that assumes that all subjects are equal even if society values them differently and provides more money for some fields and even if they require different levels of cognitive ability.

The website The Tab provides evidence from the Complete Universities Guide (still searching for the original data) that in the UK there are substantial differences in the grades required by universities for various subjects.

The five most difficult subjects measured by points for grades (Advanced level A = 120) are:

Medicine
Dentistry
Physics
Chemical Engineering
Classics.

The least difficult are:

Business and Management
Accounting and Finance
Education
American Studies
Sociology.

This is for undergraduate education in the UK. Looking at future majors of GRE test takers in the US we find something similar Philosophers, physicists and economists are very much brighter than future accountants, social workers, education specialists and public administrators. Engineers perform poorly for verbal aptitude but better for mathematical aptitude. See here and here.

Does it make sense that the average paper in a demanding discipline like physics or philosophy should be treated as exactly the same as the average paper in education or sociology?