Is there perhaps something related to smell or movement? The only time I got stung by a yellowjacket is when I pulled a Miyamato Musashi and caught one with some chopsticks. I got two deserved stings out of the experience, but yellowjackets have treated me well since. I've observed that though yellowjackets might want to land on people and investigate a while, they're soon gone as long as there's no beer involved. I just semi-freeze for a few seconds and let them do their thing. Obviously a bit uncomfortable when they land on the face or neck, but the only people I know who seem to have been stung as adults are those that are clearly panicky around wasps. I'd go so far as to qualify them as placid. Horseflies are aggressive and another beast altogether...