About once a year, I, too, experience this. I have standard training routines, and I know the typical range of my performance for each routine. And about once a year I'll finish a workout and my numbers will have jumped a good bit beyond the range. And, of course, I'll then think that I've all of a sudden I've gotten that much better. But successive workouts go right back to norm.
Trying to figure it out, I've wondered if on those particular days my bike computer malfunctioned. But it's unlikely that just on one day it would do that, plus the numbers on those days are very good, but not miraculous.