In 1977 changes were made to the striking process which resulted in two varieties, high and low 7. The low 7 is far more scarce, thus making it more valuable to collectors.
Coins are minted through a mechanical process in which die stamps are used to press images into a blank planchet to create the heads (obverse) and tails (reverse). From time to time the die stamps are worn or altered and in turn get replaced by a different die stamp. By changing the stamp this can result in slight variations of the same coin, called coin varieties.