The main shapes that make up the letters have uniform stroke width.
If you look at the bottom left of the W you can see that the letters are made up of shapes with small gaps in between. The effect of thicker strokes is caused by the fact that most of the gaps are slightly thinner than double the stroke width so the strokes crash into each other and look like a single, thicker stroke.
There are a couple of different ways to achieve this. You could use Minus Front in Pathfinder to cut thin slivers out of the letters to create the gaps or use the Divide function to create the shapes and then shrink each one using Offset Path. I would suggest that the first option has been used here because the gaps are not totally consistent.
The counters on the B and the A are a different story. They have been separated from the main letter shape (
Object > Compound Path > Release) and then coloured the same colour as the background, rather than being a hole in the outer shape.