Quantifying differences is really difficult and the commonly cited numbers are kinda iffy as a rigorous measurement (https://www.smithsonianmag.com/smart-news/what-does-being-99...) in the sense that the public interprets them - 99% comes from looking at single base pair changes only. For the coronavirus, it's an RNA virus so it'll mutate a lot faster than DNA viruses or organisms like us. And the generation time is gong to be a lot faster to allow for a lot more divergence more quickly than longer generation times in mammals.