Your handicap leaves most of your record out
Under the World Handicap System, with a full scoring record, the starting point for your Handicap Index is the average of your best eight Score Differentials from the latest twenty. Twelve are left out of that average. It describes demonstrated ability through your better performances—not what an ordinary round must look like.
Why your Index can rise after a respectable round
A Score Differential puts a score into the context of the course and tees, with the applicable adjustments. The eight lowest differentials need not be your eight lowest gross totals.
When a new score arrives, the oldest leaves the latest twenty. If that departing score was one of your best eight, its replacement in the calculation may be worse—even after a respectable new round. Your Index can rise because an excellent performance has aged out of the window.
The arithmetic is a starting point: caps, exceptional-score adjustments and Committee reviews can also affect the result. For judging progress, consider your typical scores alongside the Index. They describe different things; the best-eight calculation was never a promise to reproduce that standard every time.