Well, the Fame Rules on p. 109 (left column near top) says:
"Fame Stacks. A character's Fame is the sum of all Fame Points received."
And the stacking I was referring to in my post is not due to increases in Noble Rank, but rather of
being a Noble of a particular (final) rank (See Errata 0.71, p. 109) in addition to other sources of Fame (Military Rank, Medals, etc.), which, if I am reading p. 109 correctly, and have not missed something stated elsewhere in the BBB, means that those Fame points do stack according to the Rules as Written.
Or perhaps your comment: "I thought fame was calculated once at mustering out ... " is revealing what I am missing. Do you mean to imply that if a character has multiple careers (e.g. Spacer Officer, and then Noble), that upon mustering out of each career there is an associated "Fame Track" unique to that career, so that a character may have multiple independent Fame scores? I think this would be a fine solution, but a clarification comment to this effect should then be added to the Fame description.
I appreciate Magnus's comment upthread about Fame being calculated differently based on the "class" of people with which you are dealing (and would probably do something like that myself if I were running a campaign), but that does not seem to be what the RAW are implying. A note of clarification, I believe, is necessary.