It seems most of the the confusion comes from a) super short or super long primary received-to-secondary invite timelines and b) high-stat applicants being "passed over."
I'm sure this post will be buried under further speculation, but really, there's no point in trying to guess what goes on behind the scenes since it's pretty much going to come down to "it varies" and thus ultimately won't have any prediction value for everyone who's waiting

..except it is a nice way to take our minds off this whole process and feel like we have some control over it