Kinda makes sense, no? Looking for similarities you inadvertently go back to the common root, and you end up with something that reads like Old Church Slavonic.
it is basically "modernized" osl with simplified grammar and lexicon "averaged" from the existing slavic languages.
and, btw, osl is no way the "common root", it's absolutely not proto-slavic, just old bulgarian (from the 9th century) which happened to be the orthodox church liturgical language and thus had very significant influence on many slavic languages.
interslavic is deliberately based on osl [0].
it is basically "modernized" osl with simplified grammar and lexicon "averaged" from the existing slavic languages.
and, btw, osl is no way the "common root", it's absolutely not proto-slavic, just old bulgarian (from the 9th century) which happened to be the orthodox church liturgical language and thus had very significant influence on many slavic languages.
[0] http://steen.free.fr/interslavic/