Target heart rates are based on broad population averages. And like most 'averages' they are a pretty good predictor for about 1/3 of the population, somewhat useful for another 1/3, and a less useful predictor for the remaining 1/3. It is unusual, but not unheard of for some people to get heart rates of 200+ during intense exercise.
But the fact that you are seeing this in a LIGHT jog, and not seeing a high heart rate from brisk walking makes me wonder whether the impact and vibration of running is leading to poor contact between the unit and your skin, and causing it to report a higher hear rate than you actually have. Ensure the unit is clean and securely attached to you.
You may also want to check these heart rates against what you record walking uphill, and also whether a manual check of your pulse while jogging also produces a heart rate of 200 plus (stop jogging briefly and check your pulse - 200 bpm should give you over 30 beats in a 10 second count).
I agree with Coach Jen - the Rate of Perceived Exertion is a good guide. www.sparkpeople.com/resource/fitness_artic
But if after trying the above, you still get a heart rate of 200 bpm, you many want to talk about this with your doctor, just to be sure. In many cases this is completely normal, but in some it can indicate a problem.