Dear Saswata,
Your curiosity is quite genuine, logical, and, in fact, a legally valid one too.
Understanding the Payment of Gratuity Act, 1972
Apart from the mandatory requirement under Section 4-A of the Payment of Gratuity Act, 1972, which is yet to be enforced through notification by many State Governments, an insurance policy taken with LIC in this regard covers the employer's liability to pay gratuity under the Act when needed.
Premium Calculation and Salary Adjustments
As you are well aware, the premium is calculated and collected on an actuarial basis only, based on the average salary payable to the employee during the preceding 12 months. Therefore, there is reason to believe that any shortfall in the last drawn salary for any lawful reason would be automatically adjusted by the value of the actual maturity amount of the policy.
Employer's Liability in Case of Salary Reduction
Moreover, in the case of a salary cut during the last period of the employee's service, like the one mentioned in the original post, if the Insurer is willing to pay a lesser amount than the statutory gratuity based on a lower premium rate, the employer is still liable to compensate for the loss.
I hope this clarifies your query. Let me know if you need any further information.
Best regards