It's just that on general princeple, when facing a single 18th level character, a dragon should almost always win.
Almost always. But the dragon is mortal and at 16th level and above, the level of protection your life has is hazy. Bad luck, strong strategy, or slow reflexes can result in an inferior foe causing your downfall because at this level and higher, differences in level begin to mean less.
There are ways that anything can be accomplished. I can't think of a way to kill a fully suped-up monster manual dragon with a fully-suped up PHB/DMG only PC, but that doesn't mean they aren't out there. And there are most certainly ways if you give the PC a wider array of options.
Regardless of how "well" a dragon is played, it is still mortal, only has so many hit points and/or its saves are only so good. If sufficient offense is capable of being brought to bear against it, it can fall. Even to an inferior foe, even to lesser numbers.
Most PC's in D&D are capable of dishing out much more damage then they can take. Why? Because they have to be able to beat 3 and 4 hundred hitpoint creatures. But make the PC's take the hits they're giving out and they drop like flies.
But in any case, if a mid level character can deal out a lot more damage than he can take, a high level character can deal out even more. Sure, he can't take the same, and sure it's not the end-all-be-all of damage, but it could be sufficient to take out a more powerful dragon.