Quite simply, the big back legs on the locust, etc. were not counted as 'legs' in the same sense as the other legs. Let's use an illustration from George Orwell's Animal Farm. In this story, Snowball the pig invented the slogan, 'Four legs good, two legs bad' so as to exclude humans from Animal Farm society. The geese and other fowl objected, because they had only two legs. Snowball explained that in animal terms, the birds' wings counted as legs because they were limbs of propulsion, not manipulation.

Now note the differentiation in Leviticus—referring to 'legs above the feet' for leaping. The 'feet' are being differentiated from the 'legs above the feet' because of their difference in function. They are legs, but in a different sense than the 'four' legs which are just called 'feet.' We are being told of two types of legs: The 'on all four' legs (which are nowhere called legs; they are only called 'feet' [v. 23]), and the 'leaping legs.' It is clear that the Hebrews regarded the two large, hopping hindlimbs of the locust and the other insects of the same type as something different than the other four limbs - perhaps because they were used primarily for vertical propulsion, whereas the other limbs were for scurrying around.