here we know that hving three planes red, green and blue in 1,2,3 respectively, but here they used cr in 3 plane and cb in 2 plane. why it is used like that

function [cr, cb] = get_crcb(filename) im= imread(filename);

imycc = rgb2ycbcr(im);

lpf = 1/9 * ones(3);

cr = imycc(:,:,3); cb = imycc(:,:,2);

cr = filter2(lpf, cr); cb = filter2(lpf, cb);