I think it might have to do with how slowly the transfer takes place. from the brain transplant wiki:
"In 1982 Dr. Dorothy T. Krieger, chief of endocrinology
at Mount Sinai Medical Center
in New York City, achieved notable success with a partial brain transplant in mice.
A partial brain transplant could accomplish essentially the same goal — movement of a person's "identity" from one body to another — and thus qualify as a whole-body transplant no less than a full brain transplant."
"As Dr. Krieger demonstrated, barriers to accomplishing this feat might be much lower than transplantation of the entire brain."
Now if this were a chip and you had, say, a debilitating condition, functions of the brain would slowly be transfered over without you ever losing consciousness.
Now if a duplicate program from that were flash transmitted--THAT would be the ex nihlo copy.
I think it has to do with speed of transfer.