It's a regional thing because I fly too often to different cities and never get asked for the boarding pass but only in PBI or FLL.
Every time I tell them my boarding pass is on my iPhone which is going through the x-ray machine.

However last weekend I decided to give the phone to the agent before going through since he asked for it and he said, "what's this?" My iPhone with my boarding pass... You didn't get a paper one? At that moment I wanted to be snarky but just said no.
He proceeds to swip through several boarding passes since its open on the passbook app. "What are you doing? I ask.
I walk through the metal detector and he hand me my phone, I turn and say doesn't it have to go through the machine! he says oh yeah.

I say take it with a grain of salt, like the what's your name? Where are flying security question to catch you off guard. :