HDR brightness values change after image manipulation with pcompos and pfilt

Dear list,

I would like to both scale down the resolution and crop a circular fisheye HDR photograph of the skyvault using the following commands to do so but can't figure why I'm getting slight changes in brightness values.

The max brightness of the original HDR image is:
==> pextrem -o 11:58:00.hdr | rcalc -e '$1=($3+$4+$5)/3*179'
2.23350842
49393.2719

When I crop the fisheye image horizontally to bound the circular fisheye view evenly in the x and y dimension with:
==> ra_xyze -r -o -u 11:58:00.hdr | pcompos -o -x 3744 - -936 0 > 11:58:00_crop.hdr

I get a higher max value and the EXPOSURE header disappears from the file:

==> pextrem -o 11:58:00_crop.hdr | rcalc -e '$1=($3+$4+$5)/3*179'
1.88461284
49702.3333

Then when I scale the image down:

==> pfilt -1 -e 1 -x /4 -y /4 11:58:00_crop.hdr > 11:58:00_crop_scale.hdr

I get a lower value:

==> pextrem -o 11:58:00_crop_scale.hdr | rcalc -e '$1=($3+$4+$5)/3*179'
4.05328376
48270.3333

If I perform the scaling operation first the EXPOSURE header remains but the value is still lower:

==> pextrem -o 11:58:00_scale.hdr | rcalc -e '$1=($3+$4+$5)/3*179'
2.59250175
48056.5119

Any thoughts on what I might be doing wrong?

As always, thanks!

Chris