giblet for Gold and Platinum PC members and OWS and OWE QR operate a 'Preferred seats' function. Look at https://www.qatarairways.com/en-dk/P..._benefits.html where it says 'Preferred seats are available in the front section of the Economy cabin, which can be selected up to 72 hours prior to flight departure. Availability of seats is on a first-come, first-served basis. Availability varies, depending on the aircraft configuration and may exclude exit and bassinet seats.' .

As mihaid points out this is usually the reason for the differing availability of seats for selection.

Another practice that QR seem to be introducing is to seat the lower ticket classes at the rear of the cabin or in the rear cabin for multiple Y class cabin aircraft. This is only the initial seat allocation and has been changeable later. Now it appears that it may be becoming a fixed routine.
I may be wrong because my experience is based upon trying to place T and O booking class passengers in the Y cabins some time ago.