@flip86 My guess is it has to do with how Italians translated it. The little cakes are often sold in the “cookie” section at the bakeries. Sold by the pound along with all the other cookies. Kind of like eggs being in the Dairy section at the suparmarket. Eggs are not really dairy.
I am pretty sure biscotti or biscotto means cookie in Italian, and in America we use biscotti to mean a particular type of cookie, so it is just how the words evolved. Again a guess, hopefully someone who knows for sure will pipe in.
Some trivia: The little cakes are traditionally red, white and green layers to represent the Italian flag. They are very popular in NY Italian bakeries and the Jewish bakeries and delis sometimes sell them also. At Publix, a grocery store in Florida and other parts of the southeast, they sell the cakes around holiday times. I used to be able to buy them there all year. They sell them mini small, which is the typical cookie, like a petit fours, and also a larger version with thicker layers that they call a cake. The little ones are much tastier being soaked in almond flavor through and through, and the sweet rasberry inbetween the layers, topped with chocolate. Yum.