Yes these numbers are imperfect and as far as I am concerned improving the convertion rate on a petition is not the end goal!
Even outside Google Analytics the raw data we have in other systems is incomplete, like a volunteer could choose to register for an event using the site or just give a call to the event organizer or just show up! And that behavior will change from one country to the next.
At the end not everything is measurable: like we can’t measure our contribution to large societal change even though this is the real goal. We can’t say for example: “we are making X people more courageous to act for the environment”. But we need to ground our work in some data to at least help articulate discussions about where we are and what success look like.