Originally Posted by Often1

Forget about PNR's, bookings and the like. All that matters are "tickets".

OP had two tickets and BA required him to pay a change fee to change his BA ticket. I presume because OP has not provided any further information that the fee was in accordance with the fare rules for the ticket which OP chose to purchase.

The other passenger apparently purchased something else. We have no idea and apparently OP has no idea and never will. Without that information, we may only infer that the other passenger either purchased some kind of flexible ticket or that the agent who handled the change made an error in his favor.

Whatever the circumstance, it does not appear that OP's onward situation was handled poorly. The fact that he arrived in the morning from KUL and departed in the evening to his destination should not have prevented him from booking a single ticket.