You're question asks why the given mixed numbers are equal.
In fact, they're equal.
Lets make it easier for you to understand:
8 4/3 has a improper fraction "4/3", that fraction is over 1 whole.
You could look at 8 4/3 as:
8 + 3/3 + 1/3
The 3/3 turns into 1:
8 + 1 + 1/3
9 + 1/3
9 1/3
You would see that it turns into 9 1/3, therefore they are equal.
