You're right that's what the government says but in actuality they seem to think most circumstances are "unusual" these days, and many agencies are known for stonewalling almost all requests, throwing down
Glomar responses, and other shitty bureaucratic autism. Or even just give reasons that amount to "lmao blow me I'm the government."
And if they do that, you have to go through an internal administrative process to appeal it. And if this drags out, maybe a year or so later you've lost at all levels of the administrative process, called exhaustion of administrative remedies. And now you can sue in an actual court.
Expect to pay someone who can litigate cases like this in federal court five figures just to get out of bed.