Apple Watch S3 showing wrong distance, indoors

I use apple watch since over an year and recently bought Series 3 Wireless. The indoor measurements of distance are accurate, but when sitting in allocation, spending half an hour the distance increases some, time to couple fo 100 meters. How this magic happens?. I know, its due to movement of the arms, but it should not be the case, where GPS is not accurate to capture Lat-Long.

Why Apple is not able to correct this issue.