Recorder 6 is reflecting the NHM dictionary.   Tomocerus longicornis is a synonym of Pogonognathellus longicornis. I think the confusion here is because of the difference in terminology.

In Recorder 6 the 'Recommended Name' is equivalent to the preferred name at the NBN. Recommended names do not necessarily appear on preferred lists.  This is because the NHM can't update the preferred lists without the agreement of the owner of that list. As a result when something new happens (eg a name is changed or a species added) the List of Additional Names is used until the main list is reviewed, which may take years.  This means that Pogonognathellus longicornis appears only the List of Additional Name'.  The NBN doesn't use lists so where the taxa appears is not relevant to them.   

In Recorder 6 'Preferred Name' was in use prior to the NBN and is applicable only within a list. It only reflects the relationship within the list.  A separate table 'Nameserver' is used to point taxa to the 'Recommended' name.

The R6 taxon searching facilities pre-date the improvements to the dictionary made in recent years and do not make best use of the new tables and structures. For exampe it should be  possible to search for a 'Recommended' name across all lists. This is something we will be lookng at this year.