Last.fm does not accept listens which have bad metadata e.g. artist = "unknown artist" and the method track.scrobble in its response sends the number of accepted listens and number of ignored listens.

<scrobbles accepted="1" ignored="0">

Whereas LB currently accepts all the listens provided to it (if mandatory fields are filled), so such fields are not really important for LB responses but as this API is to work with clients which anticipate that the response will be a Last.FM type of response. And in that case these fields may be used by clients to display number of accepted listens etc.

For complete XML response check this link:

https://www.last.fm/api/show/track.scrobble