Six different brands of Italian salad dressing were randomly selected at a supermarket. The grams of fat per serving are 7, 7, 9, 6, 8, 5. Assume that the underlying distribution is normal. Calculate a 95% confidence interval for the population mean grams of fat per serving of Italian salad dressing sold in supermarkets.

the above problem yields a result of (5.52, 8.48) assuming a t distribution.

Can you please explain why t distribution is considered if the question confirms that the population distribution is normal?