PS Correction: Home basic in 64 bit version exists. I have it in my DVD stock. Sorry for the wrong reply.
Good catch, I didn't remember Ultimate had all WIMs for all editions sans Starter in the image, thanks for solving the mystery.
Windows 7 Starter x64 does not officially exist. It can be created, technically, but its use over the x86 one is limited. That's why the official x86 ISO has 5 SKUs, but the x64 one only 4.