Why did the humans say the windmill fell and how did the animals fix this?

From the text:

"Out of spite, the human beings pretended not to believe that it was Snowball who had destroyer the windmill: they said that it had fallen down because the walls were too thin. The animals knew that this was not the case. Still, it had been decided to build the walls three feet thick this time instead of eighteen inches as before, which meant collecting much larger quantities of stone."