Thank you for your reply and for sharing a really insightful article, it was a great read. I suspect that you are most probably right, and it all makes sense as I've already mentioned that I notice these discrepancies in the much older aircraft in our fleet which are over 10+ years old, versus the latest addition to the fleet everything works out spot on as per the charts and flightplans.
Also found this really interesting safety report from a B747 skipper after realising his IRS mag database was outdated and his tracks were off by 12 degrees, luckily I've never seen or experienced anything to this extent in my career!
http://www.37000feet.com/report/1203...tion-is-out-of