Last time I worked in this industry (over a decade ago) sites like expedia didn't work through a GDS but hotels would explicitly put available stock directly into them, if they had 10 rooms availble they might put 5 on expedia, 2 on another site and 1 each on 3 smaller sites. So expedia only has to check it's own data and the prices typically didn't fluctuate intra-day. I was working on software to automate most of this process.