I guess the conditions inside the cylinder are wetter, hotter and under higher pressure than what you have when you test the plug outside the cylinder. So what looks like a good spark just might not be strong enough.
Anyways, the spark plug is one of the easiest and cheapest things to replace, so it makes sense to have a few spares around just to rule that out.
In my experience, once they foul they either don't work ever again, or will foul very quickly from then on, no matter how much you clean them and how well they spark when you test them.