Just heard this explanation on the radio on the day Airbus officially launched the project and went from A3XX to A380 :
They asked marketing experts, and they found
A350 would be the normal Name, but that is a too small step in numbers for the step in Aviation, this Plane shall represent.
360 would mean a full circle, so no progress, very bad
370 is bad because the 7 is an unlucky number for our asian carriers, who should buy the plane (don't know why JAL owns Boeing 777, but never mind Boeing might have no marketing experts
)
380 makes sense, two 340 above each other (2*4=8) and looking like the cross section.
That's the outcome if others than engineers decide something