[dsp1234]

if the client tells the lawyer he robbed the bank

What reason would the attorney have for not believing the information that the client confesses voluntarily under the veil of attorney-client privilege?.

[dragonwriter]

The attorney could believe that his client is a pathological liar, among a near infinite array of other possibilities.

[gknoy]

Sometimes people lie to protect other people, or to conceal a secret that they might feel to be more devastating.

[testvox]

Insanity, duress, maybe the defendant is trying to take credit for another person's crime in order to protect them.