@JasonWalker has the correct explanation.
It is not intuitive but when you use plural relevance, you are saying there should be 0 or more of these things, but when you use singular relevance, you are saying that there must be exactly 1 of these, no more, no less.
This is why I always use plural relevance. You might write relevance on a computer with a single NIC, but then it will throw errors on those with multiple.
I always like to assume that there will be either 0 or more than 1 of the result, and plural relevance handles 0, 1, or more than 1 result all equally well.
Plural relevance effectively suppresses most errors.