Reddit user boredlurker87 mentioned describes the N-400 application processing as follows:
It is mostly an electronic workflow. When you submit the application online (and it’s true even for paper based, application is converted to online at FO), your A number is automatically validated, then scheduling/reusing biometrics and security checks happen automatically. Results are sent back to ELIS automatically and placed in file. If there’s an issue in security checks it’s placed in an officer queue for reconciling. Once everything is available, it is placed in queue for interview at the FO. The FO supplies dates, availabilities among other things to NBC and the interview is scheduled. The review occurs when the file is sent to the FO prior to the interview… other than that it’s pretty much an all-electronic workflow for preprocessing.
As a result, issues during the security checks could be one explanation of why the processing times for the N-400 Application for Naturalization Form vary so much within the same USCIS field office.
Reddit user prettyeye28 added the following (albeit no specific to N-400):
In my office there is no visibility [into who and how puts those files in shelf and in what order]. A box (or several) are emptied onto the shelf. And then we officers grab. Right now there are way too many files for the records section to look at the dates and put them on the shelf that way.
https://redd.it/vnonem seems to say the USCIS sometimes process cases out of order too (the screenshot was taken on https://hilites.today/):