A possibility is that she thinks very fast* and realizes that Harry ought to move away from the dementor (since it is effecting him some) but that it must be done in a way that makes Harry look strong, not weak. Showing that Harry has no problem standing undefended next to a dementor but walked away out of pity reinforces his strength nicely.

*= fast may actually mean that Dumbledore went back and gave her carefully edited information so she could make plans in advance