When the SAS task backscale calculates the area of a source or background region from a Timing mode observation, it currently assumes that the region is contiguous. Although it is tempting to extract a single background spectrum from regions on both sides of the source, this will currently produce a wrong value of the BACKSCAL keyword. A simple workaround is to either use a single box for the background or to extract two backgrounds individually and then manually add the BACKSCAL values. This also applies to a source region which may exclude the central brightest columns if the source is piled-up.
This bug will be fixed in the next SAS release.