Exercise Ring

Prior to my workout (indoor cycling), my Apple watch showed that I had already exercised for 8 minutes. After working out on the bike for 30 minutes, my watch showed that I had exercised for only 30 minutes total. Why did it not register the total time as 38 minutes? I'm not sure why it deleted the 8 minutes that I had exercised before the cycling workout. It's happened a few times before with my watch, so I'm wondering if there's a setting that is throwing off my numbers. Kindly advise. Thank you!