Well 4/6 is simplified to 2/3. 5/6 cannot be simplified.
Think of it like this: if you cut 2 pizzas, 1 into 3 equal pieces and then the other pizza into 6 equal pieces. If you were to eat 2 pieces of the 3 you cut from the first pizza, you would be eating more pizza than the second one because in order to get 6 pieces evenly, you have to cut them smaller than you would for 3 pieces.
