Fridge Logic: People see a lot of positive things written about a pig in a spider web and are thus impressed with the pig. No one seems to care that there is a literate spider.
Mrs. Zuckerman points this out, stating that they don't have a remarkable pig, they have a remarkable spider. Mr. Zuckerman dismisses it. The unspoken conclusion is that God put the words in the web, not some ordinary gray spider.
Fridge Brilliance: Having Uncle win the blue ribbon and Wilbur win the medal isn't just a way to tease the reader's emotions, but also a way to ensure that neither of the named pig characters has to become pork chops. Wilbur survives because he's become a celebrity, and Uncle survives because, as the local prize pig, he'll be in demand as a stud boar.
There's also a very logical reason for Uncle to have won: his size. Wilbur, having been a runt, was lucky to have even reached average size. It was likely Fern's care that let him grow even that large.