Difficulty understanding/applying the PRE function and Workflows

The current definition for PRE according to Oracle is the following: “The PRE function in Expression Builder returns the previous value of the specified field when the field value is modified but before the value is updated in the database.”

 

My current trigger event for my workflow is PRE('<plWeb_Status_ITAG>') ="Waiting for Sales Rep" when Record is modified.  

 My paradox is why does my PRE work function trigger fire when a new record is created and the value is SET to “Waiting for Sales Rep” My understanding is that the PRE value for this field before the record is changed is Null therefore it should not fire. It also should not fire because the record was created not modified.