It is as safety measure. Normally it is derated by about 9%.

In your case 1 tire load is 3550. So, when you use two tires, the maximum load would be 3550 x 2 = 7100. However, tires are nor identical, say if one tire is new and the other is worn out a bit. So, the radius is different. Tire can also wear down at different rate to each other. Thus, the load would be riding on one tire more than the other tire.

This is the reason Load Index is derated a bit in dual tire axle configuration.