Its because its continuous. 100/3 = 33.33(3) continuous, which means you can always add another 3 in the end and get closer to the lower number. Multiplying this number by 3 does not equal 99.999, it equals 99.99(continuous). You can always add another 9 in the end and get closer to 100. The reason you see this in a calculator is because it doesn't know that the number you are mutiplying is continuous.
Otherwise, 100/3 is the result of dividing 100 by 3 and when you multiply that by 3, you get 3*100/3=100. That's why you have two types of fractions.
In order to keep the math right, continuous results should always be written down as 100/3 for an example.