Net Asset Values (NAV) are usually updated everyday between 9 pm & 11 pm. NAV displayed at the time of your redemption request is as per previous day's NAV. But the NAV finally applicable for your redemption request will be as per the time of order placement. These NAVs can be different which is why you might receive a slightly different amount. 

Depending on the type of fund and the date of investment, you may also be charged an exit load for redeeming. This is a fee payable to the fund house at the time of redeeming fund units.