the answer was that it is a small turn within airway limits
Only a valid excuse if they were not on an ATC assigned heading.
These days I think that everyone has become conditioned to the traffic following the airway centerline +/- a few metres. That was unusual when we tracked NDB's and aircraft could wander a few miles left and right of the centerline even when there was no weather about.
However, on RNP routes pilots need to remember that RNP 5 means that the aircraft will be +/- 5nm 95% of the time that the indicator shows the aircraft to be on track. Unlike the VOR when one could wander from -5 to +5 degrees (a 10nm cross track distance at 60nm from the VOR) and remain "within limits", with RNP being 1 dot left of track could mean that one is actually more than 5nm from the track already!!!
The other side to this example is that if one changes course slightly to avoid a CB and regains track the other side without saying anything then in most cases, ATC will not be aware that the CB is even there. Far better to let ATC know that there is a CB worth avoiding.
Regards,
DFC