Probably because the tiles are not perfectly straight. This article explains it.

http://www.tileusa.com/Articles/RectangularTiles_Griese10.pdf

Porcelain (and maybe ceramic) tiles all have some amount of cupping in each tile that occurs during the manufacturing process as the water dries out of the tile. It is (or should be) slight but is nevertheless there. In some tiles it is significant. The use of a running bond pattern at 1/2 offset exacerbates the issue because you are putting a corner (the lowest part of a cupped tile) next to the middle (the highest part of a cupped tile). The result is an uneven floor and this is called lippage. If it is slight, a really good tile setter can work through it with minimum variance. To my knowledge, all manufacturers are now warning that a running bond should be no more than a 1/3 offset. We have had to cut down 12x24 tiles in order to mitigate the issue.

@Deborah -- minor point, but cupping is where the center is low (forming a cup).

I see your point. I have always heard it referred to as cupping but it is where the center of the tile is raised slightly. I guess that it is technically concave. Thanks.

Crown is the generic term that I'm familiar with for material that is high in the center. Lumber for example usually has a crown which is placed up when framing a floor. A road might have a crown so water drains to each side.

The important thing is as you said. Tile, especially larger format tile is rarely flat, and it's up to the installer to check the tile prior to installation.