think this is a pure neurological phenomenon. It happens unconsciously and may not directly be influenced by focus-work. The small pieces which have been imprinted by focusing have to fall together in their individual correct places. And just this may happen if you're just not focusing on them. (But it will never happen if you never imprinted them by focus-work in one ore the other way.)
Seems to me it's an analogy to learning a poem, reading and reading it again and never remember the whole piece. It might help to put the book under your pillow before sleep. But you'll never learn the poem if the book under your pillow is your only work with it...
PS: You know I'm not neurobiologist so take it at most as 2ct....