Answer
When the sizes of the sample or population are different.
The frequency of a variable of a greater sample (or population) has more chance of being greater than the frequency of the same variable of a smaller sample (or population).
Work Step by Step
Example: "How many people like soccer in your country?"
The USA population is about 5 times the Italy population. Italy is one of the most traditional soccer teams of the world. But, suppose there are 100 millions of Americans that like soccer and 50 millions of Italians that like soccer (frequencies). Does it mean that Americans like soccer more than Italians? No, because, according to the given frequencies, about 30% of the Americans like soccer while about 90% of the Italians like the same sport (relative frequencies).
