This must be because chess positions/pieces are much more unique, and on a smaller scale, than Go pieces and positions.
I think this begins to get at the phenomenon described by the OP, who mentions he is playing chess against a computer. In my (relatively limited) experience of computer chess, being computers they tend to play the same (or a similar) way each time. The upshot of this could be that if you play against the same level regularly, your thought processes get stuck in some way and you cannot beat it. By coming back several months later with a fresh approach, you suddenly make the breakthrough.
Indeed, against some chess computer programs, you can beat them more often by making objectively worse moves, if these expose some flaw in the program’s chess logic. This is true even of many world-class chess programs, as shown on Tim Krabbe’s Chess Curiosities site, on this page.
I think this is a far more likely explanation than “unconscious practice”, although it is of course not necessarily correct.
(Warning: layperson’s explanation coming up…) See various books by Gerd Gigerenzer on mental heuristics, where he discusses studies that have shown that persons who are experts in a field can have more difficulty answering a question than persons with only average knowledge, because the experts have too much information at hand and try to evaluate it all as meaningful. But persons with average knowledge of the field (and not totally ignorant) can come up with more accurate predictions. The example he uses a lot is a study where two groups of people were asked which of two cities has a larger population – a) a pair of two cities in the US, b) a pair of two cities in China, c) a pair of two cities in Europe. Americans answering the question did more poorly comparing two U.S. cities because they were slightly familiar with both of them; while Europeans or Chinese who knew of one member of the pair answered more accurately.
So I think perhaps taking a break from something for a while might be analogous to changing yourself from an expert into someone less expert, and then the simple heuristics thing starts to come into play.