It was very easy to remember seeing dogs, not so easy to remember pigs. And each one of you assumed that if dogs on leashes came more quickly to your mind, then dogs on leashes are more probable.
很容易就想起见过狗， 但好像没怎么见过猪。 所以每个人都会假设 既然能快速地想起见过狗， 那被拴着的狗就应该更多些。