Seemingly random calorie counts for exercise

JaniceLove308

I have spent about an hour browsing the community posts, but can't seem to find an answer to this, so I would appreciate any help you have. I am totally confused by the way the MFP app assigns calories for exercise. Today I logged 60 minutes of weight training, and selected the "Strength training (weight lifting, weight training)" option from the MFP drop down list on cardiovascular. I was credited with a whopping extra 20 calories. I did the same thing two days ago and was credited with 168 calories. What gives? Additionally, when I add weight training exercise any Fitbit step calorie credit I show disappears.