Sometimes there are more retro moves, mostly because of different move order.
Play 1.34-30 20-25 2.32-28 25x34 3.39x30. The Board Window shows the following position:
The Book Window looks like this:
There are three retro moves. The current position arises 10590 times through the move 39x30 and 193 times through the move 32-28.
It is not easy to see that 34-30 can be played as the last move.
To check how this is possible right-click the move 34-30 and choose Retro Line.
