Can someone shed some light over the following? I am not sure if I am missing something.

I booked a Doha - Hong Kong (Stopover) - Toronto ticket at Economy Flex (fare class M). When I booked the ticket, I checked the fare rule (the small icon next to the fare class, on the payment screen) that says,

"Rebooking
Free of Charge. Fare difference may apply"

However, when I try to change my reservation under Manage my bookings. The system trying to access a Change Fee of QAR 100. Does anyone know if this change fee corresponds to the rebooking fee listed in the fare condition pop-up window (I think they are the same thing per my experience in the pass)?