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.




This is an important point. There’s not really a “right” way to play so much as a “right way for your group”.
I don’t think D&D specifically does a good job of guiding groups into finding what they’ll enjoy. It comes loaded with a lot of assumptions, and then different players can sit down at a table without realizing how different their axioms are.