In a sense (hehheh) I agree with you. In as much as the focus on observable behavior is something that Sensing types do more naturally, perhaps our minds are not as clouded with vague theoretical notions and impressions that we can observe these things more clearly when we are looking for them?
However, I don't believe it is only
Sensors who can do this. Obviously, Keirsey himself is an Intuitive type, and I have seen others such as one of my co-workers (who is INFJ) grasp the system quite well and do a bang-up job of observing these behaviors in people she encounters, especially her fellow Idealists, who she seems to now almost have a radar for.