Fitbit Charge HR: the first four days of workouts

Howdy, friends. Today marks the 4th full day with my new Fitbit Charge HR and I'm still not sure how I feel about it overall. ;) Along the right side of this post are my workout charts from my Thursday, Friday, Saturday and Sunday's U-Jam Fitness classes. (You can click on one of them to see it larger.) I'm baffled about the heart rate readings throughout the classes. Thursday night's class was super-intense and I was out of breath frequently.  According to the chart, I spent 11 minutes in the peak heart rate zone, so that makes sense to me.  Surprised that my average calorie burn was only 11 minutes, but okay. Friday night's class was nowhere near as intense.  I wasn't really out of breath at any point, and I was stunned when I looked at the chart when I got home.  How the heck was my heart rate so much higher overall?  Peak heart rate zone for so much longer?  It really confused me - which workout was the anomaly?  Did Thursday night not get measured correctly for some reason? Well, that all got thrown out the window on Saturday.  This was another intense class - yet my "trusty" Fitbit said I was never in the peak heart rate zone!?!  I don't get it!  I sweated my butt off.  I was out of breath several times.  I definitely felt like I got a more intense workout on Saturday than I did on Friday, but I burned over 100 fewer calories?!?  I am so baffled. Then we come to Sunday.  My Sunday morning class is awesome because it's usually 90 minutes, which equals more awesome for me.  However, it had me in the peak zone for merely 7 minutes of the class, which kinda blows my mind again.  I don't disagree with the heart rates in the last half hour of class - I really was getting tired! - but I'm so baffled overall. I don't understand what's going on here, and it's so frustrating (obviously!).  I should try to wear my old heart rate monitor as well on Tuesday night (that night's instructor is one of my favorites) and compare - but it doesn't give anywhere near this level of statistics.  It will tell me the max heart rate, average heart rate and total calorie burn though, so that's better than nothing. Have you experienced such frustrations with your fitness tracker?  Any recommendations?