This really depends on sales/ fares that each of the airlines has at any given time. Maybe EK plans to fill up planes later, or is able to, while QR and EY find the need to fill up in advance, and therefore offer cheaper fares now.

A friend wanted to fly BRU-SIN earlier this summer. Initially, EY looked like the best option at under 600 euros, but the fare did not allow a stopover in the UAE. The stopover eligible fare was much higher. Then, later, EK had a fare sale to SIN/BKK/HKG and he ended up getting DUS-SIN for under 700 euros while the Etihad equivalent was 800 + euros.

It's all about inventory management. Pricing model only comes into play if we compare list fares, like GigoloArt said.