Are you serious? You're talking about a scaline effect but you dont know what a pixel is?
The pixels are what makes up the picture on your screen. A pixel is usually a square shaped point made up by three "sub-pixels" (red, green and blue).
Some pixel-technologies are LED based and each pixels can be lit individually and not only blocking light as traditional LCD screens would.
Apples "retina display" got it's name because the pixels are so small that from a certain distance your eyes will not be able to makeout the individual pixels.
(well with that logic any display could be a retina display if look at it from a far enough distance, haha
)
In this 3DS case the pixels are just that big that you can see them (and the gaps between them).
I hope that nintendos next gen handheld with have a much higher resolution than the 3DS
UPDATE: You need to read up on Scanlines and what causes scanlines on older CRT televisions.
Now, there are scanline filthers and FX to kinda replicate the effect of scanlines on LCD and LED displays but "Real" scanlines should not be present.
What you noticing here on the 3DS is the gap between each pixel row.
And I can only guess that it kind of looks like scanlines if you switch the 3d off because the 3DS may overlay the two screens sligthly off to the left and right (a half pixel or so) I dont know if this is the case but it sure would make sense.