I'm not sure what your question is..
80mm = 227px at 72 Pixel/Inch (the resolution at which Illustrator's Save for Web, saves) so I'm not sure where the problem is.
From Adobe Help Center:
"When you save artwork in a bitmap format, such as JPEG, GIF, or PNG, Illustrator rasterizes the artwork at 72 pixels per inch."
If you wish to save directly from AI at a different resolution, use
Export instead of Save for Web, It allows you to choose between 72ppi, 150ppi, 300ppi or Other (this will allow you to add
You mentioned in the comments but "really 80mm are 302.362204728px" and that is right and wrong at the same time. I have mentioned in my answer that save for web saves at a resolution of 72 Pixel/Inch. That means 80mm will have a value of 227px when exported.
Your resulted value comes from a 96 Pixel/Inch resolution that translates to roughly 302px.
You can open your exported image in Photoshop, go to Image>Image Size and you will see the resolution of your AI exported image has a resolution of 72.
Check out this calculator:
It will help you understand better.
Also, for more on the topic, check out this answer form @joojaa :