A simple example: (i will use decimal instead of binary as it is easier and binary suffers from the same stuff but for other numbers)

(10/3)x(9/4) == 7.5

but due to the fact that computers store the value instead of the fraction it would come out as something like 7.4999999999 cause instead of doing (10x9)/(3x4) it would do 3.3333333333333 x 2.25