I wish it was as simple as you try to picture that Penko, well it's not. To start with IRS deliver a true heading using gravitation equation. FMGC applies then magnetic variation dependable on current a/c position. It's not unusual for those magnetic variations to be outdated thus resulting in incorrect magnetic heading without the system detecting it. Airbus is supposed to publish the list of airdromes with outdated variations. On top of that WGS 84 non compliant countries like China only add fuel to fire as your accuracy gets screwed. It's not unusual to see being on LOC and the diamond triangle track being coupla degrees off. Let's say ILS 07 with QFU 070 with left cross heading will be in the range of 060-065 ILS 070 and track 078. Obviously a/c will align it's nose along the LOC using stored magnetic course and actual LOC signal. Ideally all 3 indications must be perfectly aligned after de crab a/c MH,MC and ILS. It's not gonna happen if the a/c's magnetic variation is wrong. One of indication for that is when the track is offset with the ILS LOC on final. Just my 2 cents. Can't find the reference though.
P.S.
And that we only need to check if the ILS course is displayed correctly.
That check is done before TOD during briefing, at least in my airline.