Sometimes middle names "appear" later in life. James Brown's son, called John after John Smith, his maternal grandfather, might appear in the parish register baptised as John Brown or on his birth certificate registered as John Brown. Later in life he might start to call himself John Smith Brown.
I have even come across a John with no middle name who went on to marry a lady with a middle name and then gave all his children middle names. Feeling left out, he awarded himself the fictitious middle name "Milton".