@kernpro mib2berlin is saying that there are actually hundreds of RSS messages you haven't marked unread...
When you see [X/Y] in the mail panel, that shows there are Y unread messages - and in those unread messages, X messages have not been shown on the screen ("unseen").
Vivaldi uses the "unread" status to keep messages in the unread views in mail and feeds (in the mail panel and using filters in the mail/feeds tabs). By default the user manually marks a message read so it will no longer show in unread views (but is still shown in other views like "received" mail etc.).
There is a setting to mark as read automatically, but I prefer to use the system manually so I keep track of my mail/feeds.