You still maintain weight if you take in less calories than you're burning.
And how, exactly, do you determine that with any accuracy? Sure, you've got the BMR formula that's essentially worked from averages, but we don't come born with a gauge that tells us what we've burned, or that when we drop caloric intake 33%, whether our body is thriving, or has dropped its BMR by 30% to compensate.
And no, I'm not being snarky here. This is exactly the problem I've run into recently. I changed my diet and dropped my caloric intake, sharply. According to the smug "calories in < calories out, simple as that" philosophy, I should have been melting weight off like a mo-fo. Instead, I just felt like I had to sleep for 13 hours a day - and I don't mean I felt lazy. I mean I came home from a normal work day feeling like I did when I used to do a 24-hours straight crunch session.