Think back to when you were single, met someone at a bar, and wanted to take that person home. You'd say almost anything (Maybe this is just me. Is this just me?)
"I'll call you"
"I like you"
"No, I'm not married."
"Yes, let's do this again." (Post)
You lied. Maybe you thought that you might call, but you likely knew you wouldn't. You did this for two reasons: (1) Because you wanted to get laid (2) Because you never thought you'd see this person again.
You know you will see your clients again but yet you say:
"We'll update you on your case all the time."
"You'll always be able to reach me."
"I feel incredibly good about your chances here. Can't guarantee, but I think we're going to come out great."
"You won't be charged more than 'x'".
Maybe you thought you might live up to these, but you likely knew you wouldn't.
How do you expect the client to feel when this happens? Probably like they just got screwed.