Apparently, the detailed maths of the outbreak indicate that it was circulating in China in November and possibly as early as October, not as currently suggested by Chinese researchers December 1 for the index case.
In addition, the discovery that infected people, including children, can be almost asymptomatic is a factor. A case in point is Dr. Chris Higgins in Melbourne today.
Then there is the discovery that the current COvid-19 tests produce a high percentage of false negatives even with repeated testing.
This led researchers to believe there are 10X more cases than reported. This indicates that the CFR is much lower than the 3% being suggested. Epidemiologists have also made the observation that overestimates of CFR are common in the early stages of a pandemic so this is not a surprise to them.