The simple explaination I have found to this "post practice improvement" is that during practice you do not realise you have "got it" and you keep practicing. It is difficult to measure when you have actually mastered a difficult passage, some people never can understand when that happens and it is only after coming back to it after a few days that they realise, oh I can actually do it.
Some people have slight inaccuracies to their phrase and it randomly pokes it head up. This randomness can sometimes cloud the perception as to when you have acquired mastery over a passage. The randomness can however be somewhat subdued after a break from practice. I also believe you can always pin point what is hindering your playing, thus there is always a clear path you can set out to master a passage, and inaccuracies become less random.
I noticed this type of post practice improvement in my own playing when I was young and constantly tried to control it. Because I don't like the idea of improving without conscious control. So then now when I practice I know immediately if I have mastered it or not, and whether subtle inaccuracies are because of lack of focus and lack of pinpointing the core reason for the mistake.
When I play a phrase I feel the general muscular memory associated with it, then I will go back and level out the inaccuracies. It is like laying concrete, you dump it in a pile, you roughly spread it out, then you make it smooth. Most people stop at the dumping and rough spread and leave the smoothing out to time.
"Complex pieces" are pieces which are full of ideas we have not much experience with. Because of this it can make progress slow. However as we relate new movements to old movements we will make faster progress. Everything new can be related to something old, that is the key to my practice of pieces with ideas I haven't seen before (which are getting much less and less as your repertoire increases)