Jet Airways — exorbitant charge for change of flight date

Address:

Mumbai City, Maharashtra

Pnr: vwvopq
Names of passengers: rakesh kumar & anuradha kumar (Both senior citizens)
Flight details:
Bom-del 07 dec 2017 9w 839 - no change
Del-bom 10 jan 2018 9w 308 - changed to 10 dec 2017 (Same flight no.)
Original fare paid: rs. 14, 314
Additional fare paid: rs. 26, 366
Total amount paid: rs. 40, 680
Please explain this exorbitant amount for change date of one flight