Being informed isn't a passive state, but an active one. One of the dangers of getting a reputation for being knowledgeable is that it creates the risk of buying into your own mythology and slacking on the activity on which the reputation was built.
Coupled with the fact that knowledge is often much narrower and more domain-specific than the reputation for knowledge that it produces, and it's quite easy for someone who has, due to time and energy invested in one domain (plus generally-applicable rhetorical skill), built a well-justified reputation for knowledge to end up turning out to be quite ill-informed (if still rhetorically skilled) in other domains in which they have an interest and choose to be an active pontificator.
Coupled with the fact that knowledge is often much narrower and more domain-specific than the reputation for knowledge that it produces, and it's quite easy for someone who has, due to time and energy invested in one domain (plus generally-applicable rhetorical skill), built a well-justified reputation for knowledge to end up turning out to be quite ill-informed (if still rhetorically skilled) in other domains in which they have an interest and choose to be an active pontificator.