Air India — Reschedule of ticket

Address:

134c vikas nagar,pattathanam

This is with regard to my ticket booking from gatwick to cochin on 1st June.
I wanted a reschedule to may 4th.The then ticket price was 44388 and when reschedule to a lower fare of 38818 .But I had to pay an extra charge of 11842 which I found as extremely unfair.Neither did I get any topup difference.
Why is it like that.
My ticket no was[protected] for my son Vinayak Sethu.I need a fair solution for this .