if site A is about gold(real gold) and site B is about wow gold(game gold), and does google think if the two site are relevant?
Probably. although Google doesn't look at just one word. It looks at the overall words in order to build up a context. however, as they are both currencies, covering earning etc, I personally think Google would view the two sites as relevant toward each other
If you use similar keywords than chances are very good that Google bots will see your sites as relative niche sites.