The gap isn't the door casings - it's between the backs of the architraves and the plasterboard. We sometimes get this because the walls aren't plumb, sometimes because the plastering is a bit shy of the casing/lining (1mm skim instead of 3mm skin). Occasionally it's because the casings supplied are a tad too deep - and yes, when installing these you should by rights plane a few millimetres off, but on new builds they're generally on price work getting a tenner (sometimes less) per casing, so if they can get away with not doing it and caulking the gap then that's exactly what is going to happen. Normally (up to 3 to 4mm) you'd just fill with decorator's acrylic caulk then decorate where it can be seen (i.e. up the sides, but often not at the top, although for draught-proofing that is a lot better). Agreed, in a perfect world it either shouldn't happen, or it should be less of a gap, though
