EXCISE AND VAT LEVIED ON?

We are in the manufacturing of biscuits.
For valuing Excise duty, section 4A is considered and on MRP less abatement and duty is paid.

Wherease for paying VAT, commercial value is computed and VAT on such sales value is arrived.

Why Excise and VAT are not valued on the same value?

Is there any case law supporting the same.