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The use of the term "referrer" is not as ambiguous as you think, though I can understand why you might be confused.
The reason the "referrer" of a Fills Out Form activity is the page hosting the form comes from the old days (Forms 1.0). It was common to use an IFRAME to embed forms, rather than the true same-document <form> element you get with Forms 2.0.
The referrer of an IFRAME (inner document) is always the hosting document (outer document) -- as opposed to the external page (document.referrer) that brought the person to the outer document. So the "referrer" constraint for Fills Out Form treats the form as if it were truly embedded within another secondary document. Even though it's not. So it's confusing, but let's say not random.
Conversely, the word "referrer" is used in its more technically proper way when you set up a hidden field to fill from Referrer Param. That's really looking at the previous, presumably offsite, page.
And as you noticed the "referrer" of a Visits Web Page activity is the actual HTTP referrer (document.referrer).
It's good to know the history of forms and that at one point in time, it was a true referrer. That helps explain the usage and clarifies things for me moving forward.
Thanks for the help Sanford Whiteman