A lot of time the trusses are set in proper alignment with each other, or are (what! how can that be?
) not all made where they are in proper alignment with each other when set.
Set one or two trusses back just a little from the ones on either side and there 'will appear to be' (because 'there is') a dip in the roof right there.
*IF* the trusses are built correctly, you will see a matching raised ridge on the back of the roof where the dip is on the front. However, if the trusses are 'not quite right' in size, the back roof slope could very well be (likely is) flat and in plane.
You would be surprised how well that telegraphs through the roof covering even when the roof covering is tile.