I would speculate that at sea level the engine more readily produces torque than at altitude. Thus a lower engine rpm setting at sea level produces the same torque as a high rpm setting at altitude for same prop angle of attack, meaning the engine at altitude would require more gas generation.

Thus a set bleed for larger overall gas generation at altitude would mean a smaller fraction of that larger amount would be taken out. That means that the torque loss would be a lesser fraction. Whereas at sea level, the little amount of gas generation needed to generate no-bleed torque would be completely taken by the bleed powering.

So if bleed took a constant 10 unit of power available and you need 100 units of power for 50% torque at altitude, the bleed would take 100-10 or the decrease would be 10% at 90 units of power corresponding to a corresponding small torque loss.

If you are at sea level and only need 40 units for 50% torque but bleed still takes 10 units, you are down 25% in power and corresponding higher torque losses.

Just my 2 cents but do not quote me on it until I got the actual formulas. Helicopters are big on torque so you might find more info on pressurized helicopters.