Apple Watch Series 2 Run Distances VERY Inacurate!!!

Good afternoon all, I hope someone can assist as I have a real issue with the GPS distances on my Watch Series 2.

I upgraded from a Series 1 purely to make use of the GPS and use the watch for running...as this would save me wearing my Apple Watch and my GPR Running watch at the same time.

However...the distances it tracks are all over the place!!!

I can go for a run at lunch time at work and do the same 6Km loop every day...yet my watch will say anywhere from 4Km to 6Km and anywhere in between!

MORE ANNOYING is the fact the map it shows me at the end IS ALWAYS CORRECT but the mile markers are just in different places.

*** See images below this text of the SAME route but showing very different distances!!! ***

Its like its using the GPS to plot the route but then using the internal pedometer (badly) to work out how far it was and the pace etc!!! Why is the GPS map correct but the distance can vary by 2Km each day on the same route!!

I have had Apple look at this twice...and I have had a replacement Apple Watch Series 2...so this is on two separate watches now!!!

I have started running in both my Watch Series 2 and my trusty old GPS Running watch to compare route info/distance/pace etc. My GPS watch is spot on every time...yet the Apple Watch is always out. I can run an officially measured half marathon in the Series 2 but at the end it will state a mile or so SHORT! SOOO frustrating.

Ive reset/flattened the watch AND my iPhone 7 Plus 5-6 times now as Apple say that's the way to fix it...this takes me the best part of a day as I have a 256Gb handset to a total flatten and restore takes AGES!

Has anyone had a similar issue and how on earth do we fix it?

Is it true that the watch uses GPS to plot the route but then uses pedometer to track distance?? If so this is shocking and makes the Watch Series 2 pointless for serious runners!!

Just to add it does this in both the Apple Workout app AND in the Nike Running Club app...I am told the Nike app just reads the data from the Apple Workout app as Apple don't give direct GPS access to third party apps so Nike cannot access the GPS and have to get round this by just reading the data from the workout app in the background???

I really need a fix to this as its driving me insane!!!!