I could not seem to find any on-line documentation on the difference between these two keys. They are definitely doing different things. For example, Left-Shift would allow me to do additional selection events, but Right-Shift would not.
In my system thy are doing the same, at least for selection and post riple.
Maybe it something with your windows setup, that the right shift is used for language selection or another program.
After a bit more experimentation, i've found the Right-Shift "seems" to pick-up everything from where the current play cursor is and where i do the Right-Shift+Click.