Apple Watch activity (exercise)

Hi guys!

I’ve had my Apple Watch for only a month now, bought it brand new but I noticed the past week that a few days my exercise minutes aren’t matching up and I can’t achieve my full 30 minute trying even when the activity so that is connected actually says 39 mins (or whatever minutes it was) directly underneath. Can anyone help me with this? Anyone having the same issue? I love those little awards and I can’t earn them if my ring is not closing after more than the required exercise time. It’s frustrating. I’ll upload screenshots so you can see.