Hello Luis,
There is one thing to keep in mind when comparing observation areas to points. This is that you are calculating R using the observation point based on the field at one specific point while using the observation area is using power values obtained by integrating over the entire area. This means that if your field has any convergence/divergence I would not expect the point and area to return the same values.
In order to move forward could you let me know what version of the product you are obtaining your results with?
Scott