A scanline effect can be seen on the on the upper screen of all 3DS models when the 3D effect is turned off (and on the N2DSXL). Because the resolution is 800*240, the pixels are not squarish but twice as high as they are wide. It is much easier to notice individual horizontal lines than individual pixels.
On the non-XL variants the effect is not as strong because the pixels are smaller (higher PPI value) The O2DS does never shows this because it has a 400*240 screen.
As soon as the 3D effect is turned on, the parallax barrier makes each eye see 400*240 pixels. The fake scanline effect goes away and the picture looks more “blocky”.