The caterers are at liberty to fix the price of the food within a ceiling limit provided by IR.
Let us consider that the entire food amounts to Rs. 100. Now the contractor can fix up the price within a certain percentage (say 7-8%) for his profit. That is he can set the price between 100.01 - 108.
This is the reason why you would find slight difference in the total fare as per the example you have presented above.