AUCTeX versions up to and including 10.0g have a bug in the calculation of the offsets for the start of a region. This affects C-c C-r commands where the start of the region does not lie on the start of a line. It also affects regeneration of single previews. To correct this, apply the patch in patches/auctex-10.patch. It might be more prudent to install a more recent version of AUCTeX, however.