It may not show "all" possible relationships, but it does show many. It shows 9 different relationships (some of them a bit silly) between my maternal grandparents.
Click the little down arrow at the right of the radio button below the 2nd person to display the additional relationships.
Also try switching the 2 people (via the Switch button) in the calculation. I get 1 relationship between my maternal grandmother and her 3rd great grandfather, if I list him first. When I switch them, I get 7 relationships, because it incorporates the relationships by marriage.