Pure random speculation, but if:
http://www.megalithomania.com/show/image/57
is sandstone, could it be that it was marked out to make a rectagular trough, but then split in half before the trough was started to be hollowed out? It sort of looks like there are marks along the bottom edge of the stone that could be the same as the ones on the upper part, but snapped off. Overall they'd make a vague rectangle.
Edit
Nah, on a longer look, the stone is totally the wrong shape for that. I'm talking out of the wrong end of my GI tract.