Overlapping indexes in Color-Map

Hopefully you can explain a characteristic of Unity.
I'm prety new to unity and trying to solve this amasing tutorial from Sebastian Lague "Procedural Landmass Generation".
Following the tutorial I stumbled across the part where he converts the noiseMap to a colorMap, visible at 6:30.
The indices are build with

Code (C#):

1.  
2. for(int y = 0; y < height; y++) {
3.     for(int x = 0; x < width; x++) {
4.         colourMap[y * width + x] = ...
5.     }
6. }
7.  

Regarding to the math, the indices shoud overlaps themself. Because for example in a 3x3 grid the point (3|0) should collide with point (0|1) : 0 * 3 + 3 is the same as 1 * 3 + 0. But the pixels are kind of displaced if I fix this issue with an additional index counter like this:

Code (C#):

1.  
2. for(int y = 0; y < height; y++) {
3.     for(int x = 0; x < width; x++) {
4.         colourMap[c++] = ...
5.     }
6. }
7.  

What's the part I'm missing here?