You are raising a good (but difficult) point here:

In the current system, the underground corridor near Berlaymont has level -1 (https://www.openstreetmap.org/way/480855836), all 4 railway platforms have -2 and the metro has -3.  That seems to be correct, because the corridor I mentioned is physically higher than the railway platform.

Sometimes we must ignore the numbering system used by operators if that is the only possibility to create a functioning routable structure. The drawback is that it makes it impossible to exclusively use OSM data to feed apps that say: "Take the elevator to level -2."

Alternatively, we can try a 0;-1;-2 system and get the landings to be at level -1.5 and so on, but we’ll need to be sure that there won’t be too many complicated cases.

Surveying more parts of this station is still on my to-do mountain.