I guess their should be a lable on the piano.. "Being human may lead to making errors when attempting to play perfectly.." ??
More seriously though, I could probably write a book to try and explain this properly (I wont, because other people already have), but perhaps a reason for a higher frequency of errors may be a result of the fact that you actually don't know the piece as well as you think...
As an example, suppose that you've memorised the piece?
Which may perhaps be done the following ways..
- Visually memorised the score
- Visually memorised the layout of the keys
- Aurally memorised the sound of the piece
- Physically memorised the feel of your movements
- Physically memorised the feel of the keys
In reality you probably used a combination of all of these, maybe some more so than others, and most importantly they fill in each others gaps - as if there are cues between them. Every method is intrinsically linked with every other method.
What if you were required to rely on one (or two, because its difficult to truly isolate them) method exclusively?
Say, if you have a DP, switch it off, and put on a blind fold. Can you still play the piece without the sound of the piano and the look of the keys cuing your memory?
Away from the piano, can you recite in your mind the physical movements and look of your hands over the keys without having a piano to actually look at?
Can you hear in you're mind the sound of every single note without physically playing them aswell?
etc. etc.
I suppose that the stronger each method is on its own the more solid the piece will be over all, - and if one method fails you are note left with a gap.
And i suppose that this is important because stray notes are probably the result of subconscious slips in any particular department.
Really if you truly know the piece you shouldn't have any sections where you are forced to think about anything at all other than the sound you wish to make. No thinking. Just sound.
*you may have to do a substantial amount of thinking in order to be able to stop yourself thinking.
...
It's also possible for the reason to be something like even though you are technically comfortable, you are totally blind to the fact that you could be far more technically comfortable. This can happen when you're tackling something that is easy enough to do with out working at it, - you are not forced to develop a "perfect" way to execute it. It can also happen if you're not super pedantic about producing
exactly the sounds you want.