Yup, it's the sun behind Earth. The shadow of the Moon is quite small and often there is only partial shadow (except for total eclipses). Look at any eclipse path, it's really thin; if the Moon cast such a big shadow everyone would get a total eclipse often.

Edit: with a full eclipse, most of the hemisphere actually does experience a partial eclipse at some point (see latest US total eclipse. The Moon's shadow is basically a blurry circle, only 100% dark in a thin spot in the center if you're lucky.