Here's my theory. Before on the SAD with all those wholegrains (funny how my wholegrains often weren't so whole) and sugar, I was hungry all the time. When I went on primal my hunger changed. I could go without lunch. I lost all my excess weight. Now it's been long enough to start to see a difference. When I get hungry, I'm HONGRY and I eat like I've killed a buffalo. And when I'm not hungry, I eat a lot less. It's like those grains and industrial vegetable oils aren't poisoning me anymore and my hunger is more real. Before, my hunger was more like coming down off a drug and needing another fix.
In other words, I wouldn't worry about eating less on one day and more on another. It's probably way more natural. If you can even eat less on that non-hungry day, in other words, try an intermittent fast, that'll even be better.
Female, 5'3", 50, Max squat: 202.5lbs. Max deadlift: 225 x 3.