ESP32 boot partition

Hi, I have set up a partition table with three app partitions; factory, app0, app1.
Works perfectly so far, even with OTA updates.
Strangely, ESP32 boots from app0 even if the flash memory has been completely erased and only the factory partition has been written to. I would expect it to boot from the factory partition.
Does anyone know this effect?
My Partition Table: