It is likely to do with lens Vignetting and the lack of image correction. The f/4.0 version of the Sigma 8mm suffered quite badly from this Vignetting issue. The much newer f/3.5 version doesn't suffer this effect.
That said, you can correct for the Vignetting with proper RAW file developing.
You may also find that using Smartblend as the blender it not as good in this Zenith area as the default Easypano blender.
This all is assuming you have shot the images with the exact same settings. All should have the same Shutter Speed, Aperture, White Balance and ISO settings.
Another way to fix this is convert to cube faces and open the Zenith cube in Photoshop and correct the area with the healing brush tool.
Anyway, what you have is nothing new. All of us have had to deal with these type of issues over the years.