WP Smush Plugin - GT Metrix highlights smushed images as needing optimising

Hi, I hope you can help. I optimised all my images before uploading. Gt Metrix was happy with these images however Google insights wasn't, so I uploaded the Google suggested image and Smushed the images. I then found that GT Metrix was highlighting the images for optimisation. I then trialled Super Smush, I was using the standard one before. I have found that the GT Metrx image seems to be larger than my uploaded image so cannot understand this. For example GT Metrix suggests I upload the optimised version for this file http://www.allaboutstyle.co.uk/wp-content/uploads/2017/05/CatherineCliffe-Stylist-MakeupArtist-10.jpg. The size of this is 118kb, yet my file is 116 kb. Every size of this image is Super Smushed so I really do not understand this. Do you know why this may be, it is the same for most of my images. I am not sure if GT Metrix is wrong or whether I need to do anything else. I am also not sure whether to Super Smush all my images. Google Insights seems ok so maybe I should leave them.
Thank you.
Catherine