The later difference is the ergodic assumption. After one repeat time the ground motion for all future earthquakes are known and the same. The uncertainty is zero. The general PSHA fails to agree and exceeds the expected value for repeat times larger than one recurrence interval because the uncertainty in the regression analysis cannot be assumed to be interchangeable with space and time. There are two types of uncertainties aleatory and epistemic.
Aleatory uncertainty is due to the inherent unpredictable nature of future events.
Epistemic uncertainty is due to incomplete knowledge and data about the physics of the earthquake process which can be reduced by the collection of additional information. The squared total uncertainty is the sum of the aleatory and epistemic uncertainties squared. In the general development of PSHA there is no distinction between these two different types and all of the uncertainty is assumed epistemic. The approach to correct PSHA is to distinguish clearly between aleatory and epistemic uncertainties. This approach seems most reasonable in the presence of large repeatable earthquakes.