When calculating the expected life of the generator, HOMER Pro divides the generator life in operating hours by the number of hours the generator operates per year.  In the aforementioned example, the generator happened to operate 8760 hours per year, but that is not necessarily the case.  If HOMER Pro is reporting that the generator with the 87,600 hour lifetime is lasting 26.7 years, that must be because the generator operates 3280 hours per year.  You can check the number of operating hours per year on the generator tab of the Simulation Results window.