I've tried to point out in my periodic updates, this is not a conspiracy or a coverup, it is the government's philosophical approach to assessment. It goes to the point that the purpose of gathering the statistics is not an end to itself, but is to inform the government's response.
El Universal has a pretty good explanation. It's in Spanish, but the text can easily be copied/pasted into
DeepL or your favorite translator.
Briefly, the government's philosophy is not to identify everyone infected. It's to use reported infections as sentinels. They then apply a "corrective factor", reportedly eight times, to arrive at the actual numbers of persons infected.
You can agree or disagree with the validity of the approach (preferably in The Pub), but it's not a secret.