The volume shown on your PerfectDraft Pro is an estimate rather than an exact measurement. It's worked out from the dispense pressure inside the keg, with a margin of around 300ml either way.
The volume also isn't calculated the moment you insert a keg. It's worked out after the first pressurisation, which usually happens when you pour your first beer.
What to try
1. Pour a beer
When a new keg is installed, the volume may not update immediately. Pouring your first beer often starts the calculation. Pull the tap handle fully down in one smooth motion.
2. Check the keg connection
Make sure the keg and tap unit are securely fitted. A loose connection affects pressure, and pressure is what the volume reading is based on.
3. Clean the machine
Build-up or residue can interfere with performance. See How do I clean and maintain my PerfectDraft machine?.
4. Reset the machine
Unplug the machine, wait a few minutes, then plug it back in. This can reset the display.