I was curious, so I looked up arc in the PostScript Language Reference [1]. It contains the following (p. 530):
> The arc is represented internally by one or more cubic Bézier curves (see curveto) approximating the required shape. This is done with sufficient accuracy to produce a faithful rendition of the required arc. However, a program that reads the constructed path using pathforall will encounter curveto segments where arcs were specified originally.