I'd agree with all of the above. I too get this, days where I'm just on it, (like you a relative thing
), and others when I just can't drive for toffee. I think it's down to two things:
a) Concentration/Focus
b) Biology - as stigpt alludes to.
Concentration/Focus -
There are all sorts of things that can affect your concentration and focus, so many things can distract. Being quick is about being in the zone, where the rest of the world just fades away and everything is tuned in to driving. The track just seems to flow and you do most stuff unconciously.
Biology -
There are simply just times of the day and days where you're body just isn't capable of the same levels of concentration and focus. All sorts of things affect it: Time, how much sleep you've had, when did you last eat and what did you eat, (carbs feed the brain - low blood sugar levels are very bad for concentration), and all sorts of other things contribute to being able to focus on what's in front of you.
Of course learning braking points, corner entries etc etc are all important, but if you're conciously having to think about this as you drive you're not going to be the quickest you can be. It's when it all becomes instinctive that you set your best times, and this requires being in the zone. I actually disagree with hyperactive to an extent. The best drivers do drive by feel, they just know from experience when/where to brake, how to enter the corner etc. They're not conciously thinking about it when they do it, they just do it.