It's true. There are several tracking problems in various excersises of YSFE 2012. My impression is that the calibration or recognition of the individual body anchor points is not always the same, apart from fluctuations, which are based on lighting conditions. Also, the spatial depth is recognized differently from time to time. So, the display of the player deviates slightly from the detected or calculated position. In some exercises, the time window for detection is set too tight and / or the tolerance for the position is too low. In addition the movement of the trainer is not totally in sync with the position to reach. Ubisoft should have done a little more fine tuning.
There are some "workarounds" for the player, for example step a little bit to the left or right side, or move a little bit backwards or forwards. In case of the right left kick in Class 2 Cardio Boxing you can get some improvement by concentrating on the acoustic signal. Try to reach the correct position just before the click comes and hold it until you hear the click. Then go on. The results should be definitely better. This "workaround" may help you in other exercises, for example slow and fast mountain climb.