how to get a considerable difference while matching two different images with a same reference image

i want to match two images and for that i am using the following code: matched_data=0; unmatched=0; z=imsubtract(a,b); for i= 1:1:300 for j= 1:1:300 if (z(i,j)==0) matched_data=matched_data+1; else unmatched=unmatched+1; end end end total_data = numel(z); total_matched_percentage = (matched_data/total_data)*100;

but i have a reference image and with 1 real time image the percentage is coming 53% and it is perfect but problem arises when i match another image with reference image the matching should be 45 5 but it is coming 52%,so in short i have to larger the difference between these two situations.