A sphere cannot be flattened without distortion. Every projection keeps one property and sacrifices another: area, angle, distance or direction.
Mercator preserves angles and inflates area toward the poles. Equal-area projections keep relative size and distort shape. Gnomonic turns great circles into straight lines.
Web maps use Web Mercator, which treats the Earth as a sphere for projection and an ellipsoid for coordinates. Convenient, consistent, and wrong by a few metres.