Apple Watch confusion re: workouts and mfp

mmghanna1

I've had an Apple Watch for a few weeks and am very confused about how mfp calculates the calories you've burned regarding workouts. Here's what I mean:

I haven't had Apple Watch workouts syncing with mfp for a while. Today I went for a 25 minute walk. When I synced mfp said I had about 800 kj left for the day (it added about 500 kj from the walk).

I decided to sync the workout from the the Apple Watch. Mfp initially said I then had about 1200 kj left for the day. About 10 minutes later it went down to about 700 kj left for the day. This coincided with the workout info appearing in my newsfeed.

Why does syncing a workout from the watch with mfp give you less kj/calories then if you don't? Why does it go up then down - but even lower?

I'm not sure what to do and what's accurate. I'd appreciate any help. Thanks.