Old Church Slavonic is the language of the earliest translations. Church Slavonic is what it became once it was being copied, read aloud and taught in places whose own speech differed from the original, and the local varieties that resulted are called recensions.
The mechanism is simple and it works through the vowels above all. A scribe copying a text pronounces it in his own way, and his hand follows his ear. Where his own dialect no longer distinguished two vowels, his copies begin to confuse the letters for them; where his dialect had lost a sound entirely, the letter for it becomes a spelling convention he has to learn rather than a sound he can hear. The reduced vowels and the letter yat are the most sensitive indicators, and a Slavist can often place a manuscript by watching what its scribe does with them.
The main recensions are the Bulgarian, the Russian, the Serbian and the Croatian, and they are distinguished by exactly this kind of evidence. The Russian recension eventually became the dominant norm for printed liturgical books, and it is the form in which Church Slavonic is read in most Orthodox churches today, which means that the pronunciation heard in a Slavonic service is a specific regional inheritance and not a reconstruction of ninth century speech.
The recensions are not corruptions. They are the record of a written language staying in use across a thousand years and several countries, which is the thing dead liturgical languages are supposed to do and very few manage.