In the experiment involving hydrated copper sulfate, overheating causes a high calculated percent value for water. why is the high reading obtained?

The answer to this question would be: Because the increase of temperature will increase volume.

Using PV=nRT formula we can see that temperature increase will cause an increase in volume. Overheating will cause the volume increase, then increasing the volume. If the percent value used is based on the volume, it will seem that the water is increased. But if the percent is using mass, there will be no increases.
That is why sometimes scientists using molality in a reaction with high temperature changes.

The answer to that would be magnesium