I mean you can enable "Trace errors" in ReDJ plugin and manage single redirects from the Error tab in the ReDJ manager.

Yes,
you can follow the documentation and enable just error tracking (you can leave track referral off if you don't need). At this point you should see some error entries in the ReDJ Manager (the component) under the error tab. And from there you can also create a redirection to a new full specified URL for each error.

The "discrepancy" is that ReDJ only records and shows the requests made on your site (since the moment you activated the feature).

So it is good to redirect only the errors you see under ReDJ (take also a look at hits counter to see if they are common or not).

Last thing, don't feel confused about the errors, anyone can call whatever he wants on any site, and you could see requests like:

mysite.com/supercalifragilisticexpialidocious

These are not YOUR problems at all. You just need to take care about errors that are caused by your site (correct if possible instead of redirect them) and errors that are caused by wrong external links (that you can redirect wherever you want without to loose the external reference).

Kind regards,
Luigi