The truth is, viruses are constantly mutating. It's what they do. It's part of the evolutionary process.
And single-stranded RNA viruses - like influenza and coronaviruses - are particularly prone to `duplication errors' during replication, and are constantly introducing mutations.
Most mutations end up being of little consequence, and do nothing to affect the transmissibility, replication, host range, or virulence of the virus. Some even prove detrimental, making the virus less `fit' than its predecessors, or attenuating its virulence.
And mutations are random (although they can be nurtured by host adaptation), and that means the odds of a virus coming up with the `right' set of mutations to spark, or worsen, a pandemic virus is a relatively rare event.
Hardly a day goes by without seeing a new (usually preprint) article or study, suggesting that SARS-CoV-2 has mutated in some significant way. These `preprint' articles are a wonderful way to get an early look at the latest scientific evidence and thought, but have not been rigorously peer-reviewed