There are two general reasons for an average person to have a higher than normal potassium value.
1. They're in kidney
failure; the doctor would look more at your serum creatinine
value rather than serum potassium.
2. A hemolyzed specimen; red blood cells were inadvertently ruptured during the collection process, spilling extra potassium into the specimen that is normally hidden inside the red blood cell.
Your doctor suspects choice #2 (the most common reason), hence he's ordering the test to be done over.