Same fare but different baggage allowance?
 

Hi,

I am looking for a return airfare from London to Bangkok. I find two websites online which offer a similarly-priced deal on Qatar Airways and on the same airfare basis, but then one confirms 30 kg allowance while the others replied me by e-amail saying it's standard 20 kg. Is this possible? Could it be due to a special contract made between the agent and the airline?

Check the airlines' websites, as I've found that it is possible for airlines to offer more generous baggage allowances for their frequent fliers.
www.GulfAirco.com has tiered baggage allowances for each FF elite level, in addition to the standard levels for each fare type, so QR may similarly be more generous if you are a FF member at checkin.

Hope this helps. :)