I have no idea but being a surveyor in our parlance azimuth is the direction (or angle from true north or south depending on which hemisphere)
Sometimes azimuth is referred to magnetic north and hence the variation between true north and magnetic north may possibly be regarded as the azimuth variation?
These are just guesses - But I would suggest that whoever wrote the report be approached for clarification of the use of this term - your question being associated with wells etc may possibly refer to the difference between the centre of the well at the top and the bottom thus giving an indication of how plumb it is ? The Azimuth variation being the direction of the one centre versus the other - in that case the report should also talk about a distance between these two points?