You can be perfectly in tune relative to yourself without being perfectly aligned with concert pitch -- is that what we're talking about here? This is not uncommon at all among folk solo performers; John Hurt is a good example of someone who played "in the cracks" but with all his strings well-tuned in relation to each other.

When you start to jam or participate in a band or group, of course, this becomes a problem.

Then there are those individuals who can play with a high level of skill but can't seem to be able to tune up at all, or even to hear how out of tune they are. Ouch!

I've noticed over the years that my ability to tune up -- actually, to *hear* the finest disctinctions in pitch -- has varied greatly, depending upon how much I'm playing and how regularly.