Don't understand "Steps Calorie Adjustment" on my iPhone

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My iPhone tracks my steps and is synced with MFP to calculate calories burned.

Today the total calories burned actually went DOWN the more steps I took.
1800 steps=65 calories
2000 steps=55 calories
2400 steps=40 calories

I'm not sure why this happened. The only thing I can think of is that it went down after I added an entry for a separate cardio exercise. I did not have my phone on my while exercising, so no steps were tracked during that period.

Anyone know anything about this?