Think of there just being 2 blocks, and ERHC getting 50% of each. Some here would add the two 50% values and claim that we have 100%. This would be very mislerading.
If you took the (50+50) and divided it by (100+100) you would get 50, which does make sense.
In our case there are 3 blocks being discussed so divide by (100+100+100)