This isn't necessarily true for all jobs, but I think it's especially true in the software industry. I've been a CTO for 17 years, but I still feel like I'm winging it. It doesn't mean I don't know what I'm doing, I have enough experience to make good judgements; but to be the best at something you often have to be on the edge of your understanding at any one point in time.
I don't remember exactly when I realised that everybody is winging it (to one extent or another), but it made it easier to trust my own judgement, it made it easier to push for something I believed in, but it also gave me a sense of how little I still know - which helps me to not get too arrogant about my current abilities.