Lump names in WAD files are stored in a very limited format. They can be up to 8 bytes long, unused bytes being filled with zeroes.
e.g. "STARTAN3" is represented just as it is, since it fills all 8 bytes. "GRAY4", however, only uses 5 bytes, so would be stored as "GRAY4\0\0\0" ("\0" representing a NUL byte).