Brad Ellis at Medium explaining when a circle is not a circle:
Here’s where the nerd part comes in, iPhone X rounded screen corners don’t use the classic rounding method where you move in a straight line and then arc using a single quadrant of a circle. Instead, the math is a bit more complicated. Commonly called a squircle, the slope starts sooner, but is more gentle. […] Overall, these decisions seem minor, but from a design viewpoint they’re fairly opinionated. Even when designers are willing to spend social capital to push these ideas, most organizations won’t put resources behind them.