How do the sizes of elements change during the rubber layout of sites?

Question

Layout

Good afternoon!
I can not understand the resizing of the elements of the rubber layout of the site. Width values ​​are set as a percentage, max.width is 1230px. At the same time, I opened the page code, and when you zoom in, in the block model, the size of the content decreases, and vice versa when you zoom out. Why is that? Or am I missing something