Thanks for your question. You mentioned you're looking for a large jacket yet you measured 28", which would be a smaller sleeve length. Did you mean 38"?
To measure for sleeve length, a common method is to measure your arm at the center point where your neck meets your shoulders. Hold your arm slightly akimbo, as though you had your thumb hooked into your pant pocket, and guide the tape down along your arm to your elbow and on toward your wrist. Stop at the point where your forearm meets the palm of your hand, basically at your wrist bone.
Give that a shot and see if the measurement seems more accurate and aligns with our size chart measurements. Also note that if you need a "tall" size the sleeves (and body) are a little longer.
If you're not 100% confident in your measurement and size, feel free to head over to your local H-D store www.h-d.com/dealer
to try on similar jackets and determine your size.