Gratuity 4.81% ???????????

gratuity calculation.
Hi,
I am a student, need some clarification about Gratuity calculation.
How in every CTC breakup gratuity is calculated @4.81%, whereas till now i have studied dat it is calculated on the basis of no. of years an employee worked......

Employee works for 26 days on an average per month. Thus 26*12 = 312 days.
As per the Act, 15 days salary to be paid for one completed year.
Hence % comes to - 15*100/312 = 4.80769230769231 which is rounded off to 4.81%.
Hope it is clear.

Dear All,
Why does companies show gratuity amount as part of CTC?
Why is 4.81% of monthly basic salary is deducted/retrieved from monthly salary?
What should an employee do if the company is deducting 4.81% of monthy basic salary, in case they are not supposed to do so?