Apple has used an existing two year old mobile chip in the development kit. It's not custom made or the one they'll be selling later.
The most likely explanation is that Geekbench (developed for x86, not ARM and running under Rosetta 2) is incapable of detecting the little cores as it has never had to look for those types. The extra cores will be available to developers on the dev kit, it's just that Geekbench doesn't report them.
