We have both glass and BB carboys that even have slight volume differences ... ie one six gallon BB that holds a little bit less than another six gallon BB ... and one Italian six gallon glass that holds a bit more than another six gallon Italian glass carboy.
Keeps us guessing ... and honest when it comes to racking and topping off. The trick, as others have already said, is to have a ready supply of top off wine ... because you never really know how much wine you are going to need to top off after a rack.
Way too many variables, including the fact that even "like" carboys hold slightly different amounts.