Final Fantasy VI allowed players to use any characters in their team barring a few missions. Rather than create seperate lines for the characters who could be there, they just used lines that could have easily been said by anyone. However sometimes you can tell that it is meant to be a particular character.
However, sometime it gets used when it shouldn't be. For example on the Phantom Train - It's always going to be Sabin and Cyan; the ghost and Shadow are optional, but most of the lines attributed to no one are more likely to be Sabin, and there's a set party here, so why is it still a random voice?