I noticed that too. Perhaps a zero fare has been issued incorrectly by the guard/revenue inspector and the original ticket withdrawn for investigation.
That's a terrible thing to do to a passenger though as it leaves them without the documentation they need to attempt to clear up the issue in the future. However, if the ticket was withdrawn by the guard, they should have a record of that and hence the OP should post the info from the web site he originally bought the ticket from so we can see what he bought...