Normal loss given as percentage of output

The output does not become 110%. If the loss is 10% of output, then for every 100 output the loss was 10 and it is therefore the input that is 110%. Since they input 13,200, the loss is 10/110 = 1,200 and therefore the output is 13,200 - 1,200 = 12,000. So it works, the loss of 1,200 is 10% of the output of 12,000.

That is correct. The input will never equal the output if some units are lost.