Don't bother with cardio machines as they have no any important details to estimate your burnt calories with accuracy. If you compare other watches then also you will see differences as they compute estimated calories using different algorithms. Remember, it's just an estimate. However, observe your HR and analyse your HR graph. HR plays major role in computation and if it falls lower than it should, the calories probably will too. I can't speak for other watches but I compare with a chest strap which never fail in reading HR and uses ECG in real time. Then, what I see is that my Ionic reads my HR with bad accuracy and very often it is way too low. Hence, the calories probably are also underestimated. If I see big difference betweer Polar app that uses chest strap and what Ionic estimated I log exercise manually taking average of both estimated burnt calories values. Anyway, because of HR Ionic always underestimates calories in my case.
