If you get out an image editor and compare where the pixels fade to the background grey, they are nearly, but not pixel perfect to the same height on the peaks of each row. Different peaks come to different heights. The darker shaded top left ones come up 2 pixels higher than the light ones.
Saying they're perfectly horizontal is slightly misleading. A sawtooth wave on a oscilloscope is perfectly horizontal also. Technically correct, but also omitting some key detail.