[neuronic]

Non-medical view: this sounds a bit like a sudden switch is happening at one point.

In COVID-19 cases, large areas of the lungs (article states 60% as an example) may be affected and only mild symptoms experienced until some point in time. Then comes the switch and you have sudden deterioration into critical condition.

In other viral pneumonia cases, more serious symptoms are subjectively experienced earlier, when much less area of the lungs is affected (article says 20% for H1N1).

As if the problem causing breathing trouble is strong in H1N1 from the beginning and suddenly switches on in the affected area for COVID-19 which had time to grow large beforehand.