How were the humans tricked into thinking conditions on Animal Farm were better than they really were?

Napoleon instructed the sheep to make comments while Mr. Whymper was near.  Often, they would remark about the increase in rations, etc.  Also, Napoleon filled the grain bins with sand and covered it with corn, so when Whymper was allowed to inspect the bins, he thought he was looking at completely stocked food supplies.  Whymper, of course, passed this information along to the other farmers.