Well, there are many things at play here... JPEG leaves it to the codec author to interpret the compressed information. You can get different JPEG implementations that will result in visually different images (on screen) when displaying the same file. Some JPEG viewers will even allow you to control some of the parameters of the render (typically, the amount of blur and the size of the "swatch" with which the blur is applied).
This is less noticeable with stills, but it's a special form of art / a matter of professional pride for teams working on video codecs: how to make pictures produced from the same file look "better".
Similarly for compression. It's far from given that two different JPEG implementations will create byte-for-byte identical files given raw image input and all the same compression settings. There's special art in figuring out what parts of the picture will compress better, what parts can use wider "swatches" etc.
It's also true that different codecs can perform better on different kinds of pictures either in the sense of producing smaller files or in the sense of producing more visually appealing picture. So, anyone trying to establish the behavior of compression of competing codecs needs to try this on a carefully selected set of images which need to include images with high and low brightness, sharp and blurred images, pastel colors and neon glow as well as pictures of things we are often interested in seeing s.a. portraits or medical images etc.
There won't be usually an all-winning codec.
Also, as for the images compared in OP: does OP know if there's any metadata written into those images? I mean, the answer can be as simple as discovering that JPEG images included a thumbnail in the image, and then all that measuring would be worthless...