What is ActivInspire?undo not erasing in order

The teacher has the students write on the board with the pen, then she makes corrections.  When she tries to erase, using the undo button on the toolbar, the last item it often erases dots on i’s and crosses on t’s that are randomly selected and are not the last item written.

 We are using Windows 7 64 bit.

ActivInspire v1/version 1.8.64868