I remember when I first started selling ceramic tile 20 years ago. Back then, shade variation was considered a minor problem. Sure, from time to time the pattern on the tile looked somewhat different, but seldom were we asked to replace a floor because the big orange circle pattern in the middle of the tile was more yellow than orange.
However, today's tiles are specifically created to be unique and to have even more shade variation than in the past. For this we can blame the influence of natural stone. Slate, for instance, has tremendous color, shade and texture variations from tile to tile. And marbles contain various minerals that change the color and structure from tile to tile.