My boss made this exact observation of me at my annual review like 2 weeks ago. I was sort of taken aback because I am really not a black-and-white thinker, but I thought about it and at least for me, the judgment had nothing to do with "thinking". He has no idea what I'm thinking, whether he believes he does or not. It's based on observable action, which makes sense because if I need to make a snap decision in the workplace, as much as I'd like to sit and ponder the pros and cons every which way, I HAVE to make a decision and stick to it. I can't afford to be flexible once I've made a decision, and hence, started on a course of action, because that would just put me back at square one. So I could very easily see how someone who observed me at work, who doesn't know me that well, could make that judgment of me.
I don't know about others, but that's how it applies to me I guess.