Hard to say what's going on there. Seems like a bunch of IL go out, then nothing for a while. They aren't all in order either - some who applied earlier, some who applied later.
My online status changed on July 30 to "in line for interview scheduling". It also says if more than 30 days have passed, to call the 800 number provided. I did call, got a reference number, and was told that I would hear from the DO within 15 days. I take that to mean 15 business days (which aren't up yet). No word back as of yet. If the 15 (business) days passes with no call back, I'll call again I guess. I'm within the 5 months so there's probably not much to glean from that yet anyway.
Some folks report receiving IL without any change to their online status, so I'm watching the mail, but nothing thus far.
The DO probably processes many different kinds of applications, so maybe they do a bunch of these, a bunch of those? I don't know how it works.
An acquaintance who applied a couple of days after me, has their interview scheduled next Tuesday, the 25th. Their application is related to being married to a member of the military so it would make sense if they are fast-tracked. They never got an IL in the mail - they found out by phone, and had their IL emailed to them last week.
I just hope that IL aren't getting lost in the mail. With the online status changes somewhat lagging, that could spell a missed appointment for somebody if their letter doesn't come, and their online status doesn't indicate that they've been scheduled.
I had called USCIS once before - to find out about doing my biometrics early. I didn't get an answer to the question (I did take a chance and walked in early and had them done in 15 minutes) - a week or so later I received a follow-up call asking me to rate the service I received from calling. Nothing like that yet after my more recent query.
So we wait. And wait.