Tempering process do increase the strength of glass many times, which makes it greater in breakage resistance, but the raw materials that are generally used in the production of tempered glass are not free from many kind of defects that includes Nickel Sulfide which creates the risk of glass breakage.
This is the major reason which forced to introduce the concept of HEAT SOAKED glass. It is defined as the transfer of potential onsite breakage in the factory under a controlled process of accelerating the inversion of the nickel sulfide inclusions to their low temperature phase. And yes there is always a probability, that the glasses that have NIS inclusions, will break during this heat soaking process.
It is true that Heat Soaking does not guarantee that breakage will be completely eliminated in installed tempered glass, it changes the results dramatically. It reduces the glass breakage up-to 98.5%.