SSIM function is not consistent.

Original Implementation of SSIM by authors is available here which i can only run gray images but built-in SSIM function can be used on color images as well. but the answer of both gray and colored is not same. can someone explain me the difference, please.

% LR image available here 'https://raw.githubusercontent.com/mugheesahmad/Fun_testing/master/LR0000001.jpg' 
% HR image available here 'https://raw.githubusercontent.com/mugheesahmad/Fun_testing/master/HR0000001.jpg' 
lr = imread('LR0000001.jpg');
hr = imread('HR0000001.jpg');
ssim(hr, lr) %colored image
% ans = 0.8433
ssim(rgb2gray(hr), rgb2gray(lr)) %builtin function
% ans =    0.7570
original_ssim(rgb2gray(hr), rgb2gray(lr)) %author implementation
% ans =    0.7574

[Hafiz Mughees Ahmad]

I raised an error at Scikit library for non consistency and they have given some direction too which can be read here. Thanks.