The missing coordinates looked real

Some of the missing address coordinates in the Belgian postal data didn't look missing. They had a latitude and longitude, both ordinary-looking numbers. The problem was that the point was in France.

The source used a projected coordinate system, where a location is stored as an x and y rather than latitude and longitude. Some unlocated Wallonia addresses used 0,0 as a placeholder. The export also supplied converted latitude and longitude columns, and those placeholders had been converted along with the real locations.

A coordinate conversion can produce a valid number from an invalid starting point. It has no way to know that zero meant "we don't have this address's location."

That matters because the Belgian postcode coordinates are calculated from the current address locations associated with each code. Averaging those converted placeholders into the result would pull the postcode away from its real addresses. Checking that the latitude and longitude cells weren't blank would have accepted them.

Those placeholder locations are excluded from the postcode results. A pair of ordinary-looking numbers isn't enough to establish that an address has a known location.

The resulting /postal coordinate is a representative point calculated from address locations, rather than the entrance to a particular building. Keeping the placeholders out means that point is based on the addresses the source could actually locate.