The precision required to determine breakthroughs is quite high.  Imagine that the algorithm is able pick out as few as a handful of seconds from thousands of seconds in detecting a breakthrough.  Small variations can play a big part in whether a breakthrough has occurred.  Here are the main reasons:
Error correction - the server removes data spikes and data errors which the Garmins don't do.
Computations have different precision - There are subtle differences on the computing architecture between the Garmin and the server with different precisio
Regardless, getting a breakthrough on the Garmin and not seeing it on the server is a tell-tale sign that Xert has your fitness signature established to a high degree of precision!
Given this precision and possibility that your breakthrough may not get detected, don't stop as soon as you hear the breakthrough sound.  Always hold on a little longer if possible. (Yes, we know .. is easier said than done!)