For myself I don't think I was smarter before, I just paid less attention to what I was doing. I didn't know about all the edge cases. I hadn't built it before so I massively underestimated how much work it would be to get done. This makes it much easier to start.

What I did before with ignorance, I now do with experience. For projects which support it, I write tests first. Find the edge cases and figure out what I'm going to skip. I will know the scope of my project before I start it.

With solid tests in place, my productivity and confidence soars. And the implementation doesn't result in as many bugfixes than they didn't in the past.

This kind of improvement is hard to notice. You're looking at the end result of your previous work and your memory of working on it will be incomplete. Instead you're looking at what it would take for you to implement it now.

On top of all of this, do you have more responsibilities or think through your actions more than you did before? This sucks time and mental bandwidth. You have less opportunity to use your intelligence.

I had the same feeling before about a story I wrote. The stars aligned for me to write something truly excellent. For years I thought that it would be my best work. I've never been so relieved to hate something. I will always be proud of it but I no longer think it's the best I can do.