Could also mean we test more people, register more mild cases.
That, and the age structure of the infected changed: the older people are doing what they can to avoid the infection, if they are able to do that.
Also, across the world, different measures are still in place, changing the dynamics of the spread, compared to the start of the pandemic when the spread was practically unconstrained. The dynamics of the spreading is also different in different settings.
There's no any scientific reason to believe that "the virus" changed in any way biologically. It's the world that does its best to adapt. Also, the schools and the universities are not opened for students at the moment in many countries, also slowing down the spreading.
Edit: answer to: "Viruses don’t mutate?"
The coronaviruses have additional mechanisms to correct the copying, slowing down the mutations, compared to e.g. flu viruses.
Edit2: answering: "The main reason for why Coronaviruses don't mutate much is because they don't have to": I wouldn't call that "the main reason", but it's a part of their success. Coronaviruses have longer genome than many other viruses, and having uncontrolled mutations in that longer genome would make them degrade too fast, so there is that molecular mechanism they have, correcting the copying errors. Additionally, they have other mechanisms to recombine their genetic material, something like "sex between (corona)viruses" where even more than two parents could be possible. But that's different from mutations and happens under different conditions. Knowing all that, and all that what est31 mentions, it is indeed true, the coronaviruses really simply don't need to "mutate fast."
Edit3: "There different strains" is not true. They are different isolates, where the completely minor differences exist, but for all it is known, until somebody proves otherwise, and nobody has, there is still just a single "strain" of SARS-CoV-2.
Edit4: Thanks to Gibbon1, yes, comparing with influenza is tricky, but maybe it's good to give readers the idea once again that this is surely not flu and that the viruses aren't the same and don't behave the same.
Viruses don’t mutate?
The main reason for why Coronaviruses don't mutate much is because they don't have to. While influenza constantly comes up with mutations so that it can come back seasonally, it seems that Coronaviruses take a different approach by evading adaptive immunity instead. E.g. feline Coronaviruses can infect cats over and over again, without large increases in ability for cats to fight it. That's also the case a bit for SARS-Cov-2, there are reports of humans getting infected a second time, but they are rather rare reports, we'd have far more of them if humans had no good adaptive immunity.
There different strains, but apparently the virulence is pretty similar [0].
[0] https://coronavirusexplained.ukri.org/en/article/cad0013/
Single stranded viruses are a lot more stable.
More testing does not explain falling CFR in South Korea for one thing.
A month or so ago, there was a research paper (admittedly an unreviewed preprint) that suggested there are different strains circulating in the US - the part I remember is that the New York City and Chicago strains did indeed act slightly but measurably differently. I think there was a third as well.
As Dr. Fauci would say, it doesn't matter, even if it were a peer reviewed it can still be bad. (1)
There are a lot of bad studies, especially on the "preprint servers" that in more quiet times would not appear at all, or which nobody would take a bit serious. Now there is a lot of wishful thinking or bias involved even in their perception.
The scientific process doesn't protect anybody from some studies simply being bad, the process is there that eventually the bad ones are going to not be reconfirmed, and the really good ones are those that have many confirmations and have even the power to make new predictions, that remain true.
There are known issues with these studies claiming different "strains" too easily.
1) https://www.rev.com/blog/transcripts/dr-fauci-hydroxychloroq...
Note that the trend is worldwide. Even in countries with very high testing in the beginning. South Korea for one!
I also think it's unlikely to have the virus pathology synchronized across the world with current travel restrictions.
Last time I checked the virus actually seem to feature some "dual gene pool" mechanics, which should make it more robust and adaptable to "fading out".