One of the mysteries of the English language finally explained.
Definition of personal service in US English:
personal service
noun
1Law the action or an act of serving process in person on a party or prospective party to legal proceedings.
2A professional service involving direct communication or contact between the parties involved, frequently with connotations of particular attentiveness or responsiveness to individual needs.
Origin
Late 16th century; earliest use found in Richard Mulcaster (?1532–1611), schoolmaster and author.