Keillor's use of this idea has an added meaning that makes it richer, I think, and not an example of the above. It is not only that everyone favors their own children just a bit and sees even the average child as something better. The opposite value is in play as well: don't be too full of yourself, don't put on airs. You're not some genius, you're just above average. When both ideas exist in tension, I don't think it's such a bad thing.
*Title stolen from Thomas Sowell.
