Why does antibody levels after a covid-19 vaccine vary between individuals with healthy immune systems?

They call them individuals for a reason. Each has a different immune exposure history from birth, which added to genetic variation will modify any immune response to future challenges. Some prior immune challenges will be similar enough to a new agent that a person has a head start in developing an immune response. Protective levels are also within a range/ not a finite number.

Immune response is under genetic control and two genetically different individuals will have different responses. For healthy diet: http://www.choosemyplate.gov/., exercise 30 minutes/day, drink enough water daily so your urine is mostly colorless, have safe sex, no tobacco alcohol weed or street drugs. Get HPV and Covid vaccines. Wish you good health!