Typically with sedentary folks (as versus those with trauma induced symptoms), the usual pattern from what I've read is tight neck extensors at the base of the neck (suboccipitals) and the upper cervical spine is in extension. This is because rounded shoulders and curved upper back put the neck lower than it should be, so we tend to extend the neck upwards to look straight and move our shoulders/arms. So as a result the portion of the neck i mentioned gets super tight. Now, there is a school of thought which says because your neck is in the position (to see a computer screen for instance) and you round your back while sitting, this is the end result. So your PT is probably attempting to fix your FHP and make you sit straight to fix things.

What other exercises are you doing for the back and shoulders and neck?