This may be quite deliberate. There's been a lot of chat about these sorts of thing here:
Time for metal chargers?
I'm not a sparky, so take what I say with a pinch of salt! It seems there's a possibility that the EV could go faulty, and the bodyshell of the car goes live, which could push current back up the PE line back into the EVSE (Newer regs & EVSEs have PEN-loss detection & additional PE contact breaker to isolate the car in it's entirety if this happens, but older EVSEs lack this feature). And at the same time, the house's PE line could develop a fault where it's no longer connected back to the grid/substation, so this combo could make anything connected to PE become dangerous. Your car located outside is not regarded as being part of the house, so needs separate Earthing, or extreme caution/special safety detection as I see it.
Maybe this is why the metal case is not connected to the House PE, but instead should (maybe?) be connected to it's own Earth Rod? If I understand what the sparkies say, the house's PE (should connect back to grid/substation) should not at the same time connect to your Earth Rod; if it did, your earth Rod could end up becoming the "safe" route to Earth for the local houses, if the grid's PE line back to substation got broken some distance away. Something like that anyway!