In addition so the above said
The IRS position is not 100% perfect, and therefore, the gravity force sensed by the accelerometers is not fully accounted as such. A small part of it is wrongly counted as horizontal acceleration. Therefore, there is a GS error. And this error increases the position error, which in turn changes the GS error, and so on. But this error is oscillatory, and the sensed positon will wander about the real position.
You can see in the ND how the airplane positon changes slowly with respect to the runway when you are on the parking stand, when the residual GS is high.
Also, when you are taxiing, you can sometimes feel you are very fast but the GS is not so fast. or the opposite. Crosscheck with the other pilot GS. Many times there is a great difference between both.
This GS drift affects the FPV, too. Sometimes while flying an ILS you can notice that the track "green diamond" is not perfecly aligned with the ILS course, even if the AP is perfectly maintaining the LOC. This happens when there is a large GS drift at right angles with the LOC course.