I actually had it wrong. Its Antenna refrence point (ARP) and not Antenna phase center (APC).
Antenna phase center is unknown for each antenna and needs to be calibrated to get the exact value. The build in antenna is an OEM model from Tallysman and phase offset could be uniq for each model, antenna phase offset highly depends on it surroundings. But its to be assumed phase center is close to ARP. A test could reveal that.
On my unites it varies between few mm to almost 4cm (reach with tin foil) and varies over time. Phase offset also changes over time.
Edit: Now that i look at the bottom text of Reach it states phase center. So i am not sure any more. Emlid might elaborate?
Edit: or its just a really good and stable antenna and phase is close to center…