In my experience, the measurements taken with these different devices are based on different time periods. The Watch measurement is almost immediate, and if you watch it, it will change frequently as you measure. I've not seen it really off too much, unless you start moving around too much, which will cause your rate to go up slightly, but short measurements of that will make it look a little higher than others.
I used to work in EMS, and when in a hurry you measured a pulse for 15 seconds, and then multiplied by 4 to get the number of beats per minute. That is assuming the rate stays the same. That is not always true. Everything else that makes measurements is doing the same thing, The only true measurement of your heart rate for a minute is to measure for a minute. If you feel yours is way off, check to make sure the back of the watch is not dirty, check to ensure that your watch is secure on the wrist.