in this site, I found the fact that Slovak and Belarusian are very closely related. the lower the percentage, the more related the two compared languages.
It is well known that Slovak and Belarusian are members of the Slavic language family, so there is obviously some degree of relatedness. Whether or not that particular website has a reasonable metric of linguistic relatedness is a matter that we could discuss at length (but we won't). Typically, such computations are based on lexical similarity, and I will assume that is there case here.
The right linguistic question to ask is comparative: is Slovak "closer" to Belarusian than it is to Czech?; is Belarusian closer to Slovak than it is to Russian? And are the computed numeric differences statistically significant. If the website say that Belarusian and Slovak are closer than Czech and Slovak, I think we would have to conclude that their computation has a problem. Also, incidentally, Czech and Slovak are not single monolithic languages, they are dialect clusters. Then adding in a geographical variable, you can ask to what extent computed closeness correlates with "distance". But of course you have to have a metric of physical distance, which is what? You probably should not pick the two most-separated spots where Slovak and Belarusian are spoken, which are about three time the distance from the nearest spots in Belarus and Slovakia.
In other words, we don't yet have a linguistic puzzle to explain.