ARF correction may be incorrect for small regions
When using an input arf file and region, the input infile image should be binned at a resolution smaller than the region size. For example, if using a circle with a 1 ACIS pixel radius (0.5"), the infile image should be binned at subpixel resolution.
Using an input image with a binsize comparable to the size of the region does not provide arfcorr enough pixels to compute an accurate model of the PSF and therefore the corrections to the ARF become inaccurate.