In the top right hand corner of the Matrix Subtotal textbox you'll see a green triangle, if you click this the properties box will populate will properties relevant to the Subtotal grey area

and now the subtotal values (which are where the grey would be) take on these properties

The Position property have 2 values, Before and After, this will determine where the Subtotal is positioned relative to the group, In the case above is set as After, which is the default, below is an example of when it is set to Before