yup it's strange, for instance Serbian and Croatian are almost the same in many things (even used to be treated as one language for a long time before the political split), but still many common words have a different gender - e.g "flu" is in Sebia "grip" (masculine), while in Croatia it's "gripa" (feminine), but "planet" is in Croatia just "planet" (masculine), while in Serbia it became "planeta" (feminine). And there are even the words like "bol" (pain) that is written and pronounced exactly the same in both languages, but feminine in Croatian, and masculine in Serbian.