There are SO MANY other reasons other than the domain name for a possible change in rank. First one that occurs is are you purchasing links and they're not? Second one is how much content do they have compared to you? Are their onsite factors better than yours? Do they have more one way inbound links than you do? Do they have more authority on this subject than you do? Are their one way inbound links of better quality from higher
PR sites than yours?
Pointing at the domain name and saying that's the reason is akin to blaming the full moon . . .