bolder is a relative font weight:
The ‘bolder’ and ‘lighter’ values select font weights that are relative to the weight inherited from the parent
bolder and lighterare even part of the official spec. How they are interpreted and displayed is up to the browser.
The fact that they appear the same visually is because most browsers don’t properly support font weight variations beyond bold and normal.
Here’s a question with background info: Are all CSS font-weight property’s values useful?