"Other" classification for epitopes in the Immune Epitope Database (IEDB)
I presume that these epitopes represent peptides that have been eluted from MHC molecules and then sequenced by mass spec.  You can then compare these peptide sequences to a reference genome (more specifically to the proteins predicted to arise from the reference genome).  If the peptide matches perfectly to a predicted protein, then you assign it that identify.  

But many peptides will not align to the predicted proteins, and so they can't be unambiguously assigned a gene of origin.  

There are many reasons why MHC-derived peptides may not match the proteome.  Most obviously, the reference genomes are typically from a single individual, so if the sequenced peptide has a SNP, or other allelic variation, it won't line up.  As well, a significant number of peptides undergo some form of modification during antigen processing, such as deglycosylation changing Asn to Asp, proteasome-mediated cleavage and re-annealing, and so on.  These modified peptides also won't line up to the predicted proteome.