Actually, they didn't say that about the 64GB version, did they? I believe they only gave numbers for the 128GB. I have found, based on the two different Samsung tablets I have, that Windows 8 reserves different amounts of space for recovery, file history, etc depending on how large the disk is. I expect you will find about 20ish GB of space used on the lower end Surface.
My current machine is a Samsung tablet with 128GB, so I assume the Surface Pro 128GB will be very similar, and there is a very easy trick to recover about 17GB of space- just use the built in tool to move the recovery media from the SDD to a USB flash drive. I don't have any idea why, but the recovery partition on mine was huge, and I was happy to get that space back.
Back to your original question, however, yes, MS really should have a 256GB version.