I've had a bit more of a think about this...
When I first pick a guitar up after a lay-off, I usually go straight to a "boogie in A" sort of thing, in a lazy "hey this should be easy" kind of way. When I'm in practice, I just flop the hand on the neck and the vibes come out...
But when I'm out of practice, applying my usual "level of attention" to the playing, it invariably sounds naff - maybe buzzing, maybe out of tune cos I've forgotten how to press the strings consistently, no attack/vibe or "life" to the thing...
So I start concentrating, and after a bit, there it is, the good ole boogie sounding like it always did... hang on! Maybe even better? Was I sounding this good X weeks/months ago?
.....
I suspect it is subconscious, but I think there's also a bit of a "reality check" on the attention/accuracy that you're putting in to the instrument. If you're really practicing hard, I suspect you'll be putting it in anyway. But for the level I play at nowadays, I get a bit lazy in how I play. And picking it up again after a break perhaps focuses things a bit better.
By the way, I also find, like Ian, that I'll start learning to do something that maybe I wanted to do before but didn't have the energy/discipline to actually figure out properly.
Mebbe it is a subconscious thing - but if it makes you play better, then it's an improvement!
