There's a 2 on the end of your second lib
The only difference I can see is that maybe libdvdcss2 is usually from a newer source version. Really, I assume the files are probably identical and the difference is preference for application similarity (libdvdread2 for example may be 'compatible' with libdvdcss2). The source is at version 1.2.9; which doesn't really correlate to either library versions. If it's an itch that you really want to scratch you may want to contact someone like the VideoLan.org folks and see if they can give you a really good answer.