One should not forget that "certified" limits often mean "tested" or "demonstrated" limits - a test pilot took the device to such-and-such a speed or such-and-such an altitude, without problems.
It might work fine a thousand feet higher or 10,000 feet higher or 50 knots faster - but no one knows for sure; no one actually tried it.
So a given engine might have been tested as a helicopter turboshaft only to 22,000 feet (and certified to that altitude) while the same powerplant configured as a fixed-wing turboprop may have been flown to 28,000 feet (and certified to THAT altitude in THAT configuration).
So there may well be no technical or aerodynamic reason for a certified limit. It can simply be "We didn't test it beyond this limit, so we aren't certifying it beyond this limit. It may work fine, but if you exceed that limit - baby, you're on your own."