This is exactly it. I think people who have the ability to dream lucidly like @Gary Duane and @-NN- probably have two things going for them: It's true that they can conjure dreams specific to their need or interest, but I think, most importantly, they can wake up at the right time. I think the fact people can exert some sort of control on their lucid dreams (this seems to be a feature unless I misunderstand the term "Lucid Dream") might indicate that the brain activity during those dreams could be a hybrid between a waking state and deep REM dreaming (so something similar to an artist in an inspired state, which kind of justifies the higher degree of coherence (if not logical at least artistic in a sense) and sometimes visual/textual/auditory sophistication. But I'm just theorizing here.