The 25k extra points can be converted in the same ratio (5:4). There is no guarantee that you will be able to book 20k worth of flight tickets using these points. When it comes to redeeming airmiles/hotel loyalty points, there is no way to determine the exact value of the benefits.
As Axis cannot guarantee a minimum possible value of reward points when you redeem them as air miles, they suggest that 25k points are worth 5k (via gift vouchers, which are guaranteed)