Perhaps it's like what Sony did with the PS3 a long time ago: they manufactured it with an eight-core processor but only allowed access to seven of them. The manufacturing yields were really low at launch so they manufactured an eight-core and played the odds that they'd be able to fix the yields later.
The PS3 had one PPE core, and eight SPE cores. One SPE was disabled for yield purposes and of the remaining seven only six were available for usage by end developers.
