If you have a chunk of potassium from a natural source it will be radioactive. Why? Because a small fraction of that natural potassium is potassium-40. The discrepancy between radiation per gram is like measuring the car exhaust from a parking lot of 8000 cars of which only one is running. Do you report the exhaust per car (the 0.000000000818 figure for potassium) or the exhaust per running car (the 0.0000017 figure from ANL for potassium-40).
Both figures are right, but they describe different situations.
You are absolutely correct, except for one little problem: Sources are citing 0.0000071 Ci/g for Potassium 40, not for Natural Potassium in quantity. Aside from the fact that the ANL fact sheet is pretty clear:
The specific activity of K-40 is 0.0000071 Ci/g.
The Plexus site in particular correctly cites the 8.54E-10 figure "per gram of natural K", but also cites it as shown below (stated units = curies/gram):
http://www.iem-inc.com/information/tools/specific-activities
Again, either the ANL booklet has a typo or there's a more fundamental problem (as published, it is either right or wrong...I lean toward a typo); a better source for the radioactivity of Potassium 40 exists...somewhere. Shouldn't it?