IRL this would not happen, because the HMS placed close to the eye, just collimated to infinity. The main cause can be the method, that the movement of the HMD sensed at position of the head, while the movement of the VR cursor and HMS symbology placed in the space further in a distance which means a given angular displacement of the head makes fairly large displacement in space at the distance of the displayed VR cursor or HMS symbology. It's more emphasized in the Apache when typing a lot of data on the keyboard. The slightest movement (pulse) can jitter it around while trying to fight it with the mouse. The same goes with the mouse VR crosshairs: because the VR cursor is linked to your head so it also moves this way. If someone only want to ocassionaly use the crosshairs then it's fine, but as long as you are starting to be aware of this problem and trying to observe these fine details then it will be obvious that something is basically wrong with the HMS crosshairs movement (including all helmet sights of the A-10, F-16, F/A-18, Ka-50 and the Apache as well).
The head tracking of the VR headsets is so precise, that even my pulse can move the crosshairs altough I hold my head very stable. It turned out, that this phenomenon is not only HMS related, it's the same with the VR cursor. Several users observed a HMS symbology jitter, which is most conspicuous with the IHADDS of the Apache.