Hi OBK
Have also noticed this on some older A320/321 but not on A330. The only sense I can make of it is the old Honeywell ADIRU variation table has been obsolete since 2000 and so the variation in it's database for given lat/long is years out of date. There is an OEB and TR which give details of airfields where you can no longer carry out autolands because of this, including some of our bases. Of particular note for my company are BFS (Jan06), GLA (Mar06), NCL (APR 07), and MAN (Jul 07), in 4 affected aircraft.
Check TR 32 in FCOM 3.01.22 page 4 for up to date list if it affects you.