Average Heart Rate and Calories Burned

MissyLibra

I just recently got a new HRM which gives me my average heart rate. I calculate my calories burn with 4 websites. Three gave me in total, 641. One gave me 494, but, it asked for my VO2, which I don't know, and they made me put 35 in case I didn't know, so I'm not sure if this one is reliable, since I didn't know the one data that changes it (without the v02, that website, also gave me 641).

Problem is, MFP gives me 508 calories burn, and all I hear about is how "MFP overestimates calories" so I'm confused.

I weight 116.6, I'm 21 years old.
I did (all back to back without break)
31 minutes with an average HR of 125 (it was mostly abs work out)
24 minutes of cardio with weights with an average HR of 148.
19 minutes of kickboxing with an average HR of 151.

I usually stick with the lower number as well, which in this case would be MFP, but I have been losing weight, DESPITE being on maintain, and eating back all my calories... so I'm thinking I AM burning more than MFP says. (aka, 1 pounds a week, despite not being in a deficit).