One possibility--the body adapts to spare glucose in a variety of ways. So, you take in a carb load on a holiday or something. This sparing could slow down the return to ketosis, the body being more frugal with its glucose stores.

After a holiday like Christmas, it can take me a while to recover and get back to normal. This year Christmas cookies had me binging on cheese and other low carb foods for a while--finally I did a day of basically fat fasting, but with unrestricted calories, and the day after that I was fine on my regular ketogenic diet.

Also Phinney has his protein suggestions quite a bit higher than what I actually eat, the lower limit of his suggested intake is at my usual upper limit, the higher the anti-ketogenic load of the diet, the slower the return should be to ketosis.