If you also have some differences between the buffering and the distance measurement methods, follow me during the investigation. If not, just read along. First, let's see the problematic geometries in context. I only have one; therefore, it is more trivial to find out the source of the problem:

The main roads are far enough from the feature; therefore, the nearest industrial area should be the cause. If we create a 500-meters buffer zone around the layer, we should see that the feature lies outside of the buffer zone. However, ST_Distance thought otherwise. What we can also see is that the buffer zones are not circular; they have sides approximating the real buffer zones. The problem is that buffers are regular polygons, therefore, they consist of vertices connected by straight segments. That is, we cannot create true circular buffers. There is no problem when we buffer straight lines, the result becomes exact. The precision problem only arises when we buffer corners, and the buffer zones should be circular. We can, however, create better approximations by using more vertices for creating these circular arcs. In QGIS, we can use the Segments option in the buffer tools to specify the level of detail, and increase the precision of the result (Appendix 1.11).

PostGIS also allows us to specify the number of segments used for approximating a quarter circle. We can provide a third parameter to the ST_Buffer function, which represents the number of segments. Let's modify the query to use 25 segments for the approximation as follows:

    WITH main_roads AS (
     SELECT ST_Union(ST_Buffer(geom, 200, 25)) AS geom
     FROM spatial.roads r
     WHERE r.fclass LIKE 'primary%' OR r.fclass LIKE 'motorway%'),
    industrial_areas AS (
     SELECT ST_Union(ST_Buffer(geom, 500, 25)) AS geom
     FROM spatial.landuse l
     WHERE l.fclass = 'industrial' OR l.fclass = 'quarry')
    SELECT h.* FROM spatial.houses h, main_roads mr,
     industrial_areas ia
     WHERE NOT ST_Intersects(h.geom, mr.geom) AND NOT
     ST_Intersects(h.geom, ia.geom);

Now the results match the ones from using exact minimum distance measurements, although the query slowed down a bit, making the other query the obvious choice.