I am not sure where you have read "files are processed in the order they arrive" . Maybe as a statement from contributors? But certainly never anything official.
For me files were never reviewed in the order they arrived. I have images waiting to be reviewed that were submitted 2 months ago, while images that were submitted 3 weeks ago have been reviewed.
And it has always been like this for me - now with AI images flooding the queue and the insane review times it just has become more extreme and therefore more noticable.
But even before it used to be like this and single files were always sorted out - For me for example it was usually images of objects isolated on white. Even though I submitted them in a batch with images of the same category (commercial, photos), the others were always reviewed 1-2 days earlier than the ones with objects isolated on white.

My guess was always that whenever a reviewer was unsure about whether he/she should approve the image, the image would either go to a different group of (maybe more experienced?) reviewers, or it was maybe thrown back into the pot, because the reviewer simply didn't want to deal with it and then the waiting time for that image to be reviewed was reset. Not sure my explanation makes sense.