I think I've found the problem with horizontal banding, at least for my color Horizon; others might be different.
The banding seems to tie to the Offset used. The fault Gain/Offset values, per ATIK support are:
...Low: Gain 1, Offset 17
...Med: Gain 6, Offset 29
...High: Gain 30, Offset 139
When I enter the above values in the Custom selection I get distinctly different results than when I select Low, Med or High. It almost appears the Offset being applied by the Low, Med and High is not being used or is lower than stated; thus, giving rise to horizontal banding of various degrees.
This is an instance where light pollution is actually a benefit by setting an artificial Offset; thus, imagers with lots of light pollution will probably have no problem with horizontal banding. However, imagers in a dark site may see banding!
Until this problem is resolved by ATIK Support I would suggest using the Custom selection and setting your own Gain and Offset values. You can use the ones above or select them from the graph (PDF file) I've attached.
Enjoy!
bwa