Why is the mobile end with REM rounded corners

REM is calculated based on the size of the Web page and, of course, through media queries.
For example, if your design is 720px wide, then your text will be the original size divided by 11.25, which corresponds to the following media query 720px, such as 16px will be 16/11.25 to calculate this. Before using the following code to put in your CSS file:
@media only screen and (max-width:1080px) {
HTML, Body {
font-size:16.875px;
}
}

@media only screen and (max-width:960px) {
HTML, Body {
font-size:15px;
}
}

@media only screen and (max-width:800px) {
HTML, Body {
font-size:12.5px;
}
}

@media only screen and (max-width:720px) {
HTML, Body {
font-size:11.25px;
}
}

@media only screen and (max-width:640px) {
HTML, Body {
font-size:10px;
}
}

Why is the mobile end with REM rounded corners