Why a 25-hour average engine life was no guarantee

Was a 25-hour engine lifespan adequate if an aircraft was only expected to survive about 20 flying hours?

An average lifespan does not guarantee that an individual engine will last that long; it can fail much sooner or last considerably longer.

Comparing the quoted survival time with average engine life hides the variation between individual engines. A 25-hour average leaves open the possibility of a much shorter life, so it does not assure the pilot of 25 usable hours. An individual engine could also last considerably longer.