Why were there only 20.6 hours in one Cambrian period day?

Answer 1

No one knows or can verify the number of hours in a day during the Cambrian Period. However, it is believed by Geologists that the Earths rotation was slightly slower before the Permian extinction and the Beginning of the Mesozoic Era. A large asteroid which may have been responsible for the Permian extinction may have also caused the Earths rotation to increase in speed and may be responsible for the "Chandlers wobble" of the Earth.

Answer 2

In the Cambrian period, the Earth was spinning faster due to tidal forces, causing days to be shorter. The 20.6-hour day duration is an estimate based on the geological record and astronomical models.