In your case, the state that mattered was stored inside eMMC. eMMC contains two small boot partitions that can be used to hold the bootloader independently from the main storage partition and to implement a simple A/B scheme. PureOS does not use them by default; apparently the image you flashed did and enabled booting from one of those partitions.
This is a standard thing lots of eMMCs implement and there are standard utilities in both u-boot and under Linux that let you configure it.