The volume reported on XRP is taken from BTC.Bitstamp/XRP only. There are more markets trading XRP out there. Also this is coinmarketcap.com not cointradevolume.com.

Maybe make the selection of the "Filter Non-Mineable" change the URL? coinmarketcap.com/only-mineable.html or something like that...?

Though a lot of XRP still hasn't been distributed, in theory they can be spent and traded any minute - all of them. This is different to e.g. Bitcoin, where only coins that were mined can be spent/traded.