Patrick:
On 4-5 occasions, my best round on one of my own courses was my first. I had visualized the shots to be played so many times, and mostly visualized executing them successfully, that it was easy to put those thoughts into action. [It could also be partly that the greens were slower and it was easier to save pars after missed greens.]
Once I've played the course a few times, and missed a bunch of shots there, the confidence wears off.