All flatbed scanners have minor inconsistencies in their scanning mechanisms.
It should then come as no surprise that there are areas within the imaging area of your scanner that are better than others. To determine your own sweet spot simply make a scan of the entire image area using a clean white surface to scan. Set the resolution low, say 72 spi to 100 spi. Once in Photoshop, use the Equalize command to exaggerate any minor differences within the image area. As you can see, my own scanner has dark spots on the edges as well as a bright streak on the left.