Modern MacBooks models have rounded corners at the top right and top left corners of the screen, see [0]. The bottom corners don't do this and external displays don't do this. This is a stylistic choice to match the curvature of the laptop's edge. If you move your mouse along that corner it doesn't disappear behind the curve, it follows the curve.

[0] https://www.apple.com/v/macbook-air/x/images/overview/mac-pl...