I am not sure what you mean by the east west and south slavs. This were after all individual tribes that had similar langauages but they are similar in concept to the various germantic languages.

Such as high and low german or dutch and german, or even english and german. So to me it is not so strange to think they have different developments.

Also, I know your aware of this but others might not be.

There are more tribes than Polish, Russian, Czech, slavock. There was like the Welds (I think thats the spelling) and other groups (esp. between the Oder and Elbe) that we tend to forget about as they became absorbed as state building was advanced.

And some groups (like around Novograd) might be considered not to be slavic.