Yes, a nullable Boolean field can be confusing because of the, let's say, overly generous way Boolean values may be interpreted in client apps (which is where it matters most).
In a database itself, a nullable Boolean-type field can have 3 values, let's say
- something representing true
- something representing false
- something representing no value yet or value unknown
When written as above, the 3 values have clearly different meanings and seem like they could/should be acted on differently (think about opt-in, where not having answered the question is different from having answered with "No").
But in reality, that 3rd value -- the "no value" value -- may be presented to the client app (say, a JavaScript web app) as an empty string, which isn't exactly the same as the literal value false but in a loose comparison it will be considered the same. (We call that a "falsy" value.) When the value is sent this way, there are effectively 2 possible values, not 3.
Or that "no value" value may come into the app as a true null -- which represents its value in the database with total accuracy -- but it still may be used in loose comparisons only, where it is
more like false than true, hence also "falsy." And again, there are effectively 2 possible values, not 3.
Or, most confusingly, "no value" may appear in the app realm as a literal false, because it's been "massaged" on the server side to always be either true or false.
So what you're seeing in the Activity Log is the actual transition of the value in the database from no value to one of the 2 other values. But that doesn't mean that while the value was still at no value that you would've been able to act on it differently from false. In most Marketo contexts it would've been the same as false or no.
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