What is the reason why a browser can render text and images with subtle differences?

Question

Browsers

Hello.

I'm writing a framework for visual regression testing.

There is an actual screenshot of the site page and the expected one.

There is such a problem - on some pages of the tested site, the actual screenshot may change, namely, some images or text may differ slightly.

As I understand it, the problem is at the level of the browser, the system, the rendering of the site. For what reason is this happening?