Fitbit charge 2 heart rate accuracy ANOMALY

[T1M0THY]

Issue:
Recently (this year 2019) I have noticed that during steady exercise my heart rate reading will occasionally jump up approx 20beats for around 20/35 seconds and then return back down. e.g. Walking on treadmill at 5km/per hr with 5% incline during warmup/cool down my heart rate is around 95-105bpm. When my heart rate is measured using a chest strap it remains steady over a 10 minute period, however whilst wearing my fitbit at the same time the fitbit readings will occasionally jump from 100bpm up to 120bpm or even 136bpm during this 10minutes period, maybe a few times. 

I wondered whether this was me or my device so got my wife to try walking on a treadmill for 10minutes wearing both the chest strap and fitbit at the same time. Oddly, the same thing happened - the fitbit reading sometimes jumped up +20-30bpm whilst the chest strap gave a steady reading.

Note: During these measurements, the fitbits had factory fitted straps, the devices were worn in same place during test, pace, breathing, temperature were all kept at a constant. Chest strap was worn at the same time as fitbits.

Question:
Are fitbit readings more accurate than my chest strap - which if it is the case I (and my wife) could both be potentially be experiencing arterial flutter (or some other abnormality that I should look into). However, I think this is unlikely as most would agree that chest straps provide better more accurate readings. 

Alternatively, are fitbit readings really this inaccurate having such a tendency to overestimate calories burned. e.g. Going on a long hike for 8 hours with potentially several +20-30bpm fluctuations within every 10 minutes would result in readings suggesting far more calories burned than what is actually burned. 

Was wondering whether something has changed between surge & charge compared to Charge 2, or whether an update has effected something as I never noticed this anomaly before. 

About us and our usage:
I'm male, 40years, 182cm, 89kgs, eat a plant based diet (vegan), enjoy moderate weekly exercise and weekend hiking. I've been using fitbit products for a number of years (started with the first generation fitbit surge) and now own a fitbit charge 2 (for around a year). My wife (female, 30years, 170cm, 62kgs, same diet, moderate weekly exercise and weekend hiking) also has a charge 2. We use fitbit because of it's simplicity over chest straps, but do own and use those too.

For interest I've attached a comparison of a 1hr walk on a treadmill. The top graph show the fitbit reading (heart rate reading goes as high as 156bpm) and the bottom graph show the suunto chest strap reading (heart rate reading doesn't go over 107bpm).

[MJS19]

I'm using a Charge 3 (have had the 2 and others previously). I know the Fitbit is wrong because I counted the beats myself over timed periods. Will explore non-Fitbit alternatives for my next tracker.

[T1M0THY]

@MJS19 thanks for feedback. Yeah, fitbit the charge 2 and app is great to get motivated and start taking notice about lifestyle. They are also useful as a reminder and rough guide, but now I realise they are not accurate enough to compliment sports training or fine tuning diet plans.

I used to use Suunto watches with chest straps and always found them reliable, but changed to fitbit for the convenience. However, suunto now do built in wrist monitors.

Since increasing my workout time, I've now gone back to using a Wahoo Tickr (bluetooth with ant+) coupled with my phone to record heart rate with apps (movescount, ismoothrun - and/or old suunto watch) to get a better picture of how many calories I'm burning so I can more accurately manage my food intake without over eating.

Unfortunately, it seems as if fitbit greatly overestimates calories burned during activities so it's far too easy to eat too much. 

[Ma0Ma0]

I was starting to wonder as well, as I took an evening walk today, and my heart rate according to fitbit went up to 170 bpm. When I counted my pulse it was around 100 bpm. So what is happening?