RPGMemes

Bob presumably has been using player knowledge to inform character decisions in a way the group doesn't like.

For example, illusions may require a wisdom check to realize they're not real. When Bob rolls openly on the table and gets a 1, he decides as a player that his character is going to treat the lava monsters as illusions. If he instead had to roll in the opaque jar, he as a player would be less certain about if they're illusions or real.