I have a similar experience with music. In my mind I can hear hugely complex compositions but in practice I'm nearly tone deaf and am incapable of transposing these sounds to the outside world. When I do write music it is always spontaneous and improvised with an instrument in my hands. I agree these must be different systems and I'm sure we can come up with other such divisions -- the gap between understanding meaning and expressing it in words, for instance. I suppose child prodigies are those with a natural bridge between these systems?