Short sector flights = fast align.
Longer sector (3+hours) = full alignment.
Drift is the reason.
Next time after a flight have a look at your POS as indicated on screen and in FMC.
If any of the positions indicate a small but significant shift from the GPS known GATE/PARKING position then do a FAST realign.
If any of the positions indicate a large shift from known position, execute a FULL align to get rig of nonexistent ground speed which will be displayed on FMC pages whilst parked.
Nail on the head right here, SJ.