I have a corresponding campaign that says:
Data value changed = unsubscribed then change data value Marketing Opt-in-1 = False
Do you see an issue with methodology?
Well, you have a checkbox control whose unchecked value is being ignored even if the end user is trying to send "No."
Of course, in standard HTML, there's no such thing as an "unchecked value" because checkboxes are defined as only existing in the form post data if they are checked. Unchecked checkboxes have no value -- they are not considered "successful" in W3C parlance -- and are not posted.
Marketo Forms turn this rule on its head by having single checkbox fields always post a value ("Yes" or "No"). So that's a drastically different meaning.
And based on the form field label, leads will make assumptions about which meaning is in play.
Ultimately, I think this comes down to Marketo Booleans being a problematic control type, esp. for fields that are more like four-state fields (not known / no, don't send me / yes, send me / now stop sending me).