FGR#11 is interesting and gives some insight into how this bureaucratic world functions. A couple of things I noted. The table of p28 shows a good dozen elements (including Ca) where the values changes by well over and order of magnitude from on "model" to the next. The following summary on p29  says: "for about 70% of radionuclides, the differences are not substantial, ie., less than a factor of four. 

That points out that none of these figures are derived from actual physical measurements but from computer models. Also there is no qualification on the uncertainty of these values as one would expect if it was a scientific test (which it is not , it's a model) . The magnitude of the changes and that fact that a factor of 4 is seen as "not substantial" gives an idea of how reliable the number may or may not be. 

http://health.phys.iit.edu/extended_archive/9503/msg00092.html
I don't know what the exchange rate for K is, but it is prsesent in signif-
icant amounts in the urine, at concentrations that vary comparatively widely.
This would suggest, I suppose, that the body regulates K very closely, disposingof excess amounts very rapidly.  Perhaps someone should do a study by feeding
Reference Bananas to....  [his dots]

http://health.phys.iit.edu/extended_archive/9503/msg00074.html

I have found this "Banana Equivalent Dose" very useful in attempting to
explain  infinitesmal doses (and corresponding infinitesmal risks) to members
of the public.  (Interestingly, the anti-nukes just HATE this, and severely
critisize us for using such a deceptive concept.)

Would love to go into more detail, but have to get back to our DEADLY Human
Radiation Experiments (i.e., eating bananas.)

Considering the fact that the DOE has officially stated that "there is no safe
dose of radiation" my advice to you all is to stop eating immediately.

Disclaimer:

Neither Lawrence Livermore National Laboratory, the University of California,
nor the Department of Energy recommends eating bananas.

So it seems the advocates of this BED idea do regard it as a bit of an insider joke, and take joy in using it to be provocative. 

But if the K-40 in a banana is 400 times more potent than that already in muscle tissue there is still a "substantial" problem somewhere. 

At least this is getting closer to understanding where it comes from and how it is calculated.