I think that is natural for some people... you're just not supposed to get as sick as a full blown episode of the flu. Also, the shots only cover what "they" believe to be the most common or likely flu strains for the "season".
I've had flu shots, started sometime after I turned 50. I never got sick after one of the shots.
I've had the flu before, though rarely in my life.
I haven't had the flu since getting the shots.
Do the work... or am I just lucky?
it depends on your body, where you live, probably is the whys. I have told by people, that don't get flu shots. it doesn't work on them. maybe depends on the flu strain on your body, if you get sick or not.