Quick answer
Because the rating is really a heat-removal budget. The same 1100 V copper XLPE cable is rated differently laid direct in the ground, pulled into a duct, or clipped in air — and the pattern reverses with size. A 4 sq mm cable carries 47 A in the ground but 41 A in 40°C air, while a 400 sq mm carries 537 A in the ground and 737 A in air: small cables shed heat well into soil, but for large cables the soil's thermal resistance becomes the bottleneck and free air wins. Always size against the rating for your actual installation method — our LV XLPE current-rating chart lists all three.