Hi there,

This is related with the room space definition => The room dimensions (label) show the dims between the axis of the walls. The wall to wall dimension tools show the distance between the surfaces of the walls.

A workaround:
1 - Click in the center of the room => deactivate the label (Name, Dimensions and Area)
2 - Go to the Detail tab => using the polygon shape => outline the desired area => select the polygon => Properties bar => Select Calc Area + Auto-Dimensions
3 - Using the Text tool => Add the desired name, dims and area.

Hope this helps.