Trevor, the simple explanation is that this is a recognised feature of John's writing style. Throughout his gospel he uses agape and phileo interchangeably.
Notice that Jesus describes the Father's love for himself as 'agape' in John 3:35, but 'phileo' in John 5:20. Jesus later describes the Father's love for his disciples as 'phileo' in John 16:27.
IT's not just these two words that are used interchangably. If you read through John in the original language, he constantly interchanges synomyns, and uses different phrases that mean the same thing. When you see this, it becomes much clearer that this issue is one of John's style.