I think the reason is that they use several different shades of the same colour close together. For example, the top right square of that sample of the pattern has 7 shades of green, 4 shades of brown and 1 black in those 900 stitches. And it's on 18 holes to the inch fabric while a lot of cross stitch is 14 holes per inch, and the smaller the stiches the more detailed it looks.
It is the most complicated pattern I've ever done in 30+ years
It is the most complicated pattern I've ever done in 30+ years