they have that, because they ask if you were prior in the past in a relationship, so you put all your relationship history in their forms, that's why they know.Interest to sponsor form (back in 2019) did not ask for co signer at all. It only asked you to provide the names of parents/Grand parents and your PR/passport details but they did not ask about the co signer at all.
So either you have your partner names somewhere else in cic documents and that is why they treated as family of 4.
I will suggest to check all the documents you submitted to CIC and definitely, it has a section where you declare that you have/had spouse/partner so you might made a mistake in declaring it properly.
Also, the note above is correct that if you separated in 2023, your income requirement should meet for the family of 4 for the years 2020,2021 and 2023.
I will suggest to seek a professional help but I just wanted to clarify that family of 4 is not coming from co-signer in the interest form.
All the best.
She was not a co-signer in the official 2023 application.