I understand writing in Italic is a way to express direct internal thought of a character, but what if a narrative is written in a normal font but it looks like a character is having thought from a first person perspective?

For example:

Karras lifted a hand to his brow. He knew he needed sleep. But he couldn’t. Now now. What facts? The tapes at the Institute? What would Frank find? No. But who knew? Regan hadn’t known holy water from tap water. Sure. But if supposedly she’s able to read my mind, why is it that she didn’t know the difference between them?

Extracted from The Exorcist.

The phrase and sentence written in Italic are definitely from a first person perspective since the author used present tense, so the character is having that thought in his head. The sentence without italic looks like it’s also from a first person perspective, yet it is not written in Italic. What’s the difference between them? With Italic and without Italic?