When you order an A3 leaflet that is folded down to A4, the "finished size" of the leaflet will be A4. This means that the leaflet will be printed on A3 paper and then folded in half so that it measures A4 when finished.
The finished size of a folded leaflet is determined by the final size of the folded leaflet, not the size of the paper it is printed on. So in this case, the finished size of the A3 leaflet that is folded down to A4 will be A4, regardless of the fact that it is printed on A3 paper.