Heart rate using activity app on I watch is always very high - up to 205

[iMore Question]

I am 60 year old female and 5'4" and 110 lbs - always been very active - I am a runner with a 175-180 cadence (steps per minute) - it seems that my watch is putting me at a higher heart rate - it goes as high as 205 and I am only going at a casual run pace (12 min mi pace) - as soon as I stop and walk it goes down to what I consider a normal heart rate for walking - my resting heart rate is around 65
I beginning to wonder what exactly is this tracking? My armswing? I would not consider myself a "super" athlete! Please advise!
I've been using this app and i-phone watch for around 5 months

 

[Peligro911]

I am 60 year old female and 5'4" and 110 lbs - always been very active - I am a runner with a 175-180 cadence (steps per minute) - it seems that my watch is putting me at a higher heart rate - it goes as high as 205 and I am only going at a casual run pace (12 min mi pace) - as soon as I stop and walk it goes down to what I consider a normal heart rate for walking - my resting heart rate is around 65
I beginning to wonder what exactly is this tracking? My armswing? I would not consider myself a "super" athlete! Please advise!
I've been using this app and i-phone watch for around 5 months

how tight is the watch on your arm ?
maybe its getting a bad reading. it read the heart rate using sensors on the bottom of the watch itself.