Nothing unusual about this.
Likely you have some combination of the wind pointing your airplane at an angle to your track - as calculated with your wizz-wheel; your wet compass being out of adjustment; your compass adjust card missing or you're not making the adjustment and possibly some transient interference with the wet compass (metal glasses case on the glare shield caught me out once).
Also your GPS could be then issue - if your magic magenta line is showing 'true' rather than 'magnetic' course.
Remember also that the 'numbers' for the runways were probably set years ago - the do change with drift but rarely get repainted with the new ones. For example mt home airport, TBPB, has a single runway 09/27 - but 09 is actually about 75 degrees - the airport I learned to fly at KHWO also had 09 runways but in the past year or so due to drift these are now 10L and 10R. So while setting or checking ones altimeter by reference to the field altitude before take off may be a good idea, verifying/checking the set of ones wet compass is not.
Have all or a number of these effects all acting in the same direction and it would explain the issue; either way getting the wet compass checked would be a good idea.
A further potential cause - acceleration/deceleration dip and swing - another effect to add to the others - not an issue if one is flying straight and level at a constant speed.
The wet compass, the DI and GPS will never all agree but big differences should be investigated.