How can a 0.5 molal solution be less concentrated than a 0.5 molar solution?

The answer lies in the units being used.

"Molar" refers to molarity, a unit of measurement that describes how many moles of a solute are in a given volume of solvent (measured in liters). By comparison, molality evaluates moles of substance per kilogram of solvent. 

Say, for example, we had 1 mole of a solute, and we were dissolving it in 1 liter of solvent, which weighed 2kg. The molarity would be 1/1, or 1, and the molality would be 1/2, or 0.5.

So, as long as the volume and the mass of the solvent are different values, we will get different molarity and molality measurements for the same solution.

Back to the original question; how can the same numbers result in solutions that are less concentrated in molality than molarity? The answer requires a little wordplay. 

In both cases, we have half a mole of solute. That aspect is pretty straightforward. The concentration is another way of talking about density; how many particles of solute i find in a given volume of solvent. In the less concentrated solution, I should find fewer particles of solute, meaning they are spread out more. Therefore, this tells us that the 0.5 molal solution must have a greater volume than the 0.5 molar solution. In fact, if we were to evaluate the 0.5 molal solution in terms of molarity, it would have a lower value, and that would confirm our expectation.