Why is cake flour different than other flours

Additional helpful information on this point can be found here: http://www.seriouseats.com/2016/05/why-no-unbleached-cake-flour.html ;o)

Cake flour is at the bottom end of the gluten scale of flours. High gluten flour (think bagels) tops the scale at 14-14.5%. Next is bread flour at 12-12.5%, followed by all-purpose flour with 10-10.5% protein, or gluten. Pastry flour, difficult to find unless you're a bakery and can order it wholesale, drops in at 9%, and cake flour closes things out at 7%. Recipes that call for cake flour will be significantly more tender if you actually use cake flour as opposed to all-purpose flour. The difference between 10% and 7% may not at first sound significant, but the difference in protein content is nearly 30%. That's a lot. Think cake versus cookie. You can use a fork to lift up a bite of cake, but for a cookie you need to use your teeth.

And be sure to sift cake flour. It is so finely milled that it is notoriously clumpy. I just use a simple basket sieve and push the remaining lumps through with my fingers.

It contains less gluten than bread or AP flour, so it produces a tender product rather than a chewy product.