I tried to improve my Google Page Speed Insights mobile score by optimizing images on the website. I analyzed the website a few times - when I didn't do anything and when I replaced some not optimized images what was supposed to make things better. Each time I got a different score. First it was 49 or even 34 (when I didn't do anything) and then when I changed some images I got 54, 52, 49 or 34. It doesn't make much sense. Why does this happen and what can be done to improve the score?