When purchasing Sunkist citrus I noticed some numbers on the sticker. Different produce items have different PLU code numbers. The store where you purchased the fruit uses these codes for their checkers to be sure they are ringing up the correct item on the cash register. The thickness of citrus peel is dependent upon annual variables, the most critical being weather. Extremely hot or cold temperatures, excess wind and the degree of rainfall can all affect the thickness of peel.
Thicker peel is nature's way of protecting the quality of the fruit's interior.
There's a bonus effect if this occurs. Bringing fruit to room temperature yields the most juice, especially when it is rolled on the counter with the palm of the hand before squeezing. Grate extra citrus peel and freeze in sealed plastic bags. Use as you would fresh grated peel. Navel oranges contain a component that turns the juice bitter when frozen, so it is not recommended.