You cannot give others commendations if you queued as a group, so it's possible that the people you've encountered have queued as a group and cannot give commendations out.

Not only that... but I've noticed that tanks tend to get a LOT of commendations if they don't mess up. I've been queuing with my tanking friend (as a healer) and he gets the majority of the commendations, even though I am doing my job just fine AND dpsing with decent gear. I think people appreciate a tank that really knows what he or she is doing because that means the run will be smooth. The player reputation system is supposed to encourage good sportsmanship, kindness, and education... but I feel it is interpreted more as an MVP system. (They actually changed the name from MVP because they didn't want it to be interpreted that way.)