For airlines, price differences are usually attributable to different fare buckets. I would look at the fare buckets and things like cancelation, change and seat selection policies. That's usually where the differences are. The 5xx might be a basic ticket with the most restrictions. 750 has some flex and 780 the most permissive is what it likely is. Not all sites will operate on all platforms... it could be other reasons as well but that would be my first guess.