Why is it that some days they (the fingers) just work better than others? Those of you who play regularly and / or perform know what I'm talking about. You're on stage, doing a tune you've done a zillion times, and when the time comes to solo, if you're feeling confident you go for the flashier licks, and if you're not, you play it safe and keep things simple. (Stage fright and nerves not being the issue here.)
Obviously, the key is in achieving such a level of consistency through practise, that only you yourself know when your fingers "are working" and when they aren't, especially if you're a stage performer.
But why is this? If you're a seasoned player, shouldn't you be "on" all the time, assuming you've reached a higher level, from practising? I'm not afflicted with arthritis or any transient, inhibitive problems with my hands, yet I can always tell within 5 minutes of picking up a guitar whether "it's happening today" or "it isn't".