Why is there a drift value between the IRS position and the FMGC position present even after performing a full IRS alignment on the A320?
TLDR: The FMGC position is a mix of GPS and IRS (called GPIRS), but the IRS position is aligned to the airport reference coordinate, not to the GPS position.

The difference you observe is between the FM position and the MIX IRS position. This difference is called the bias.

Position Computation
Each FMGC computes its own aircraft position (called the &quot;FM position&quot;) from a MIX IRS position (see below), and a computed radio position or GPS position.
The FMGS selects the most accurate position, considering the estimated accuracy and integrity of each positioning equipment.
GPS/INERTIAL is the basic navigation mode provided GPS data is valid and successfully tested. Otherwise, navaids plus inertial or inertial only are used.

(Airbus A320 FCOM - Auto Flight - Flight Management)
The FM position will move towards the GPS position in flight, if GPS is available (otherwise to the radio position):

FM Position
At flight initialization, each FMGC displays an FM position that is a mixed IRS/GPS position (GPIRS).

At takeoff, the FM position is updated to the runway threshold position, as stored in the database, possibly corrected by the takeoff shift entered on the PERF TO page.
In flight, the FM position approaches the radio position, or the GPS position, at a rate that depends upon the aircraft altitude.

Note: The FM position update at takeoff is inhibited when GPS PRIMARY is active.

Bias
Each FMGC computes a vector from its MIX IRS position to the radio or GPIRS position. This vector is called the &quot;bias&quot;. Each FMGC continuously updates its bias, if a radio position, or a GPIRS position is available.

(Airbus A320 FCOM - Auto Flight - Flight Management)
The MIX IRS position is determined from all three IRSs:

MIX IRS Position
Each FMGC receives a position from each of the three IRSs, and computes a mean-weighted average called the &quot;MIX IRS&quot; position.


(Airbus A320 FCOM - Auto Flight - Flight Management)
I think your confusion comes from the fact that you think the IRS is aligned with the GPS position and therefore MIX IRS and GPIRS should agree directly after alignment. This is however not true:

Initialization
The F-PLN origin airport coordinates are extracted from the FMS database. These
coordinates appear on the MCDU INITA page, and are normally used for initialization.
They are the airport reference coordinates.
If a high navigation performance is desired, (i.e. for long-range flights without GPS
and without radio navigation updates, or if low RNP operation is expected), the crew
should adjust the airport reference coordinates to the gate coordinates, provided that
this data is published or available on board.

(Airbus A320 FCTM - Normal Operations - Pre Start - ADIRS Initialization)
As you can see the IRS is aligned against whatever is in the MCDU INITA page (airport coordinate by default), not GPS. Therefore, the IRS position can deviate from the FMGC position (GPIRS), even right after alignment.
This deviation will remain in flight, only the FM position will move towards GPS and the bias vector will describe the difference.