I've wondered about that myself. Since you brought it up I decided to
investigate a bit. Here are a few things I noticed.
GeneSeq (Nucleic Acid/Peptide Databanks) (11) each have about 31,000 sequences.
GenBank Patent (78) has about 2,700 sequences.
GeneSeqN (Nucleic Acid db) starts with N00000 and ends with Q40111.
GenBank Patent starts with A00001 and ends with I07267.
And lastly, I did a few test searches and sequences which I found in GeneSeq's
database where not found in GenBank's and sequences found in GenBank's were
not found in GeneSeq's (that I really don't understand).