how matlab function dct2() is different from the the following 2-dct code

i applied matlab function dct2() and the following code. both methods gives me different values. if you explain why? it will be helpful. thanks

T = dctmtx(8);

dct = @(block_struct) T * block_struct.data * T';

 codeI1 = im2double(codeI); % coded image
B = blockproc(codeI1,[8 8],dct);
mask = [1   1   1   1   0   0   0   0
        1   1   1   0   0   0   0   0
        1   1   0   0   0   0   0   0
        1   0   0   0   0   0   0   0
        0   0   0   0   0   0   0   0
        0   0   0   0   0   0   0   0
        0   0   0   0   0   0   0   0
        0   0   0   0   0   0   0   0];
B2 = blockproc(B,[8 8],@(block_struct) mask .* block_struct.data);
figure
imshow(B); 
invdct = @(block_struct) T' * block_struct.data * T;
I2 = blockproc(B2,[8 8],invdct);
figure
imshow(I2);