To get the best quality print when screen printing, we need as flat of a surface as possible between the pallet (that the garment lays on) and the screen (that holds your design). The fronts and backs of t-shirts are consistently smooth and the same thickness, which allows us to get a perfect print every time. However, when you print over a seam, pocket or zipper, it creates an inconsistency, because these are thicker portions of the garment. When you print over these portions of the garments, it creates a gap in the print where the ink from the screen does not touch the shirt due to the ridge caused by the seam. Here is an example: