In December, 2020 Arceus-Dark runs 16% of the time Serious Nature in its spreads. However, it doesn't carry any mixed sets. So what's the reason of this? And more surprisingly, all of Serious Nature suddenly disappeared in January 2021. Is it more of a glitch or anything?
Source December 2020's usage stats.
Now in January 2021, the latest usage stats say that most of the sets have Calm Nature. What caused the significant change in nature usage? Plus, this is for the highest ladder available, >1700. Which means that it's less unlikely for weird sets, so is it a error in Smogon's usage stats or anything else?