Hypothesis: IRCC wants to determine how the Citizenship Act applies to your chain to make you Canadian. Exactly how.
In retrospect it seems like a trivial and obvious thing to say. But IRCC hasn't explicitly said this. I have no way of verifying it for certain. And a lot of people (myself included) have been operating under a different assumption up to now.
The Citizenship Act is a very convoluted beast. Over the years there have been a lot of changes to how people acquire and lose Canadian citizenship. Court rulings and legislation have rectified a lot of the historical injustices with a series of "Band-aids", as I call them, reinstating citizenship retroactively to our ancestors.
From our vantage point today, it seems quite straightforward: if you descend from a Canadian, you're Canadian, provided there are no adoptions and nobody was formally stripped of their citizenship by the Canadian government before the next generation was born. But that apparent simplicity is the end result of a complicated mess of laws. Depending on the specifics of your chain, who had what nationality and when, that is going to affect which Band-aids apply to your chain to make you Canadian. I now think that matters.
Why do I think this?
• CIT 0014 asks us to provide "proof of other nationalities acquired by you and your Canadian parent before February 15, 1977, if applicable to you". It seems very odd that they would only care about this for you and your parent, and not anybody else further up the chain. But the wording makes perfect sense when you consider that the form was originally written before Bill C-3, at a time when only G1 applicants were eligible. I think they've neglected to update the form, and that's giving us the wrong impression of what they want from us. I think they want proof of pre-1977 nationalities for the entire chain, not just you and your parent.
• IRCC's internal communications contain a lot of talk about how applications are to be categorized and processed based on which paragraphs of the Citizenship Act apply (for example, look at pages 63-65). It stands to reason that this categorization process becomes difficult, if not impossible, when applicants don't clarify the circumstances that caused their ancestors to historically lose Canadian citizenship.
• A recent approval where a non-urgent G3 got a very quick decision, based on an application containing nothing but certified birth records and a marriage certificate for the female G0. At the time, Canadian women who married American men automatically got US citizenship and lost their Canadian citizenship, so that's all IRCC would need to determine exactly which parts of the Citizenship Act to apply to this chain. Note that non-urgent G3+ is an extremely rare type of approval. I'm not sure, but this may be the first one we've ever seen that hasn't been linked to a G1 or G2 who also applied and got approved.
• The seeming randomness of who is getting sent to PSU and who isn't. It's been very difficult to suss out any pattern, with similarly situated people going to PSU or avoiding it with little rhyme or reason. We've even got urgent applicants lingering in PSU for ages. If some people are providing various ways to tell their G0's non-Canadian citizenship (naturalization records, some census records, and marriage certificates for women of a certain time would all indicate this), while others are not providing anything of the sort, that could explain the discrepancy. It's the sort of non-obvious pattern that wouldn't have jumped out to us analyzing the data, since it depends on the specifics of each chain and its documentation, including things we haven't been asking about like what gender everyone was.
Why haven't people been asked to provide additional documents then?
This is the most obvious retort to my hypothesis. If IRCC really wants this information, why wouldn't they just ask for it? Because we're talking about historical injustices that were rectified by the courts and the legislature.
Like, they've determined it was wrong to deny someone citizenship based on their gender, so I can see why IRCC wouldn't want to say "hey, since your ancestor was a woman and this Band-aid potentially applies to you, you need to provide extra documentation or we're not going to process your citizenship application". That seems like a messy can of worms to open. There would certainly be litigation. Also, if your Canadian G0 never became a citizen anywhere else after they left Canada, there isn't any way to demonstrate that, yet that would be relevant.
I can see why IRCC would want to apply the exact letter of the law to each application, rather than just saying "well all these differences got ironed out in the end so it doesn't really matter", but I don't know how they can accomplish that. It would be awfully tempting to kick that can down the road and focus on the chains that do have a clearly defined pathway through the Citizenship Act under their established procedures.