Most probably your mapping is optimized for just that rangeActually I was wondering - all the cars I've ever owned takes a while to adapt to new octane numbers. And during the adaptation period, the cars FC and performance takes a dip or is somehow effected. After about 100km or 2 days of driving it settles down and feels better.
Did you give your car the chance to adapt? Since you said after changing back to 97 the power came back. It sounds like your car was tuned for 97 only? Higher octane doesn't mean better power, since it's harder to ignite the timing needs to be adjusted to compensate which'll probably result in more power?
Some cars don't have clever adapting ECUs like the MMC ECUs too. So perhaps this is the issue with those cars that didn't experience the power increase? Or perhaps some engines just can't utilize anything higher than 95/97 like my pathetic Nissan Sylphy.