18 lb-in at what setting (just curious, at 7% error it sounds as though you were near the top of its range)?
There is a small port on the wrench body through which you can calibrate the HF wrench, pull off the plastic cap and there is a calibration screw:
"Clicker" torque wrenches, except the beam type, need to be calibrated at least once every two years--more frequently if often used. It is also important to back off the torque setting after using the wrench. I back it off all the way and then tighten the adjustment just up until you can start to feel resistance--this will leave the torque mechanism engaged, but the spring minimally loaded.