Celestia confused power with ability to direct that power. Celly has more power in total (ability to raise the sun/or not, ability to send people to the moon, etc, but that doesn't mean the power can be brought to bear the same way as someone else's power. Celly lost to Chrys because she forgot that. Like how we humans can destroy any animal on the planet with our weapons, but if we get into an arse kicking contest with a porcupine we're in for a world of hurt (in this case a pony powered by love and whose main weapon is the power of love, tried to defeat with weaponised love a creature who feeds on it).

Or like in Greek mythology various less powerful gods would get the better of more powerful ones from time to time depending on circumstances and personal domains.