Ammi Jaan kahti thi Railfanning se bada koi dharm nhi hota or koi Railfan chhota nhi hota - Shanzil Kabir

Hi Friends,
According to railways website senior male citizen concession is 40% (ie 60% payble fare).
I was just trying to workout the concessional fare for train no. 12880 BBS to BSP in 2A.
[[12880 BBS LTT SF EXP 3-11-2011 BHUBANESWAR BILASPUR JN SRCTZN
Fare/Charges...
more...

Class -- Second AC
Base Fare = 846
Reservation Charges = 25
Superfast Charges = 30
Other Charges = 0
Total Amount = 901
Concession Amount = 306
Net Amount = 595 ]]
Now 595 is 60% of neither 901 nor 846.
In fact 595 is 66.03% of 901 and 70.3% of 901.
Thank God! Atleast Railways is not charging, say, 55% of fare instead of 66.03%. But what's the discrepancy here? I know there is something I'm missing but what's that?
Please suggest.
Thanks to all.