Remember that 25,000' chamber runs (and their 40,000' predecessors) are normally done with young, healthy military pilots, not older, sedentary airline pilots. Response to high altitude is VERY variable with physical conditioning, or lack thereof.
While I once thrived at 8,000', I can feel the effects of 6,000' these days. Even 100% O2 at 23,000' was a bit of a chore 15 years ago...