Growing up without privilege is (obviously) markedly more difficult than being provided with the best education money can buy throughout childhood.
The students aren't necessarily worse; but they will be unaccustomed to the codified approach that other students from independent schools understand.
The system has been built to serve the privileged.
While you might feel blame can fairly be placed on differing entry requirements; the truth is more complex.
A 'sticking plaster' solution has been lazily applied to address disparity, when in reality, the whole system needs to be reworked.
'Dumber' and 'worse', are not labels that should be used here.