My son is 8 months old. I remember during the first several months it seemed like his levels were very unstable. The doctor, dietitian, and other PKU parents all told me that it was very common. When levels are very high they will be given more of the PKU formula to quickly drop it, knowing that once it drops they will begin slowly decreasing the amount of PKU formula. They want to drop it quickly when levels are in the 1000 range so that the levels don't stay there for long. However, they want to raise the levels slowly so that they avoid having them spike. Babies go through a ton of growth spurts (when they utilize more phe) and go through periods of eatting less, or more. That is why babies get blood tests done so frequently.
We have moved twice in the last 8 months and have had three different dietitians in three different states. Each has been very willing to help me understand the methods behind what they are doing. Don't be afraid to ask your dietitian any questions like this that you have. Pick their brains! You will be glad you did.