I don't know the answer to your question. Sorry...

But in Dr. Snow's paper, he addresses neuro problems in patients in his study with appearing "normal" blood levels of B12 who improved with treatment anyway:

http://neurotalk.psychcentral.com/post698522-70.html

There is a "pump" in the spinal cord that moves B12 into the CSF, from the blood serum. It only works well the the levels are high. This is why low serum levels, lead to neuro symptoms since B12 cannot get into the spinal cord and brain.
MS and Fibro have had studies showing low B12 in the spinal fluid.

When B12 is injected according to my reference here AHFSDrug Information reference...it only lasts in the serum for about 72 hrs. It is then lost thru the kidneys or soaked up by other tissues.

I seems that there could be a problem in some people with how B12 is managed. But proof of that is not easily forthcoming.