Here is an example. Let us assume compund X is 90% pure and dissolves in water at the rate of 50 g/l. It contains Y which is soluable at 100 g/l as an impurity.
Therefore, 50 g of X contains 5 g of Y.
If you take 100 g of XY mixture and place it in 100 ml of water, you will dissolve 50 grams of X and ALL OF Y. This means that the solution is now a saturated solution of X which is now 10% impure. By using bad procedure, you have doubled the impurity.
If you are after the solid, it is more pure than that in the liquid, but if you assume a third ingredient Z which is less soluable than X, then you have concentrated the Z in the solid as well.
Do you understand?