Wouldn't the HRM be more accurate if watch is worn on the inside of wrist ?

As has already been discussed in numerous forum posts - it depends.

As it didn't come up in numerous posts when I was typing.

The wrist is a pretty poor place to measure HR - whether on the inside or outside. (Bony, poor circulation on an extremity of the body, subject to sudden movement).

It is however a convenient place to VIEW information, which is why people wear WRIST watches in the first place.

Thanks.

if you are thinking that the watch would sense or 'see' the pulse from your radial artery- that's not how optical HR sensors work.  they shine a light into the skin and sense the reflection from just below the surface- and then use some processing horsepower to find the variation- which is caused by capillary blood flow just under the skin's surface.  that said- the skin on the underside of your wrist, may be less sun damaged, thinner and have better capillary flow.  It doesn't seem to be any better for my wrist- but worth a try for yourself.  Every wrist is different.  

I believe so, yes.  I seem to recall a few years ago, some company (Fitbit or Apple) got some bad press because their HR monitors were not working so well on dark colored skin.  Garmin probably had the same issue but they don't get the headlines the way Apple or Fitbit does.  I assume that they addressed it by changing wavelength of light.  Tattoos are also supposed to be really hard on optical heart rate sensors.

Thanks .

They haven’t changed it. Read the Garmin support notes about WHR. And Suunto. And Apple. And Polar And...anyone else who is honest about the limitations of optical heart rate from a wrist watch.