This is because if you add 2/3 together until you get that amount, it will add up to having three people. If this sounds confusing, here is a visual thing to help.
2/3+2/3=4/3 which can also equal 1 1/3
1 1/3 + 2/3 =2
you added 2/3 together three times to get two, so three is the answer.
