Zyxel GS1900-8hpv2 flash memory space showing 4MB total while techdata states 16MB

I understand that Zyxel GS1900-8hpv2 has dual-boot functionality which effectively halves the available flash memory space. However, I would still have 8MB total. What I am seeing after executing df -h however is rom having 2.5MB and overlay having 1.6MB, totaling ~4MB, with the remaining being ram memory. Does the device have quad-boot or something?

The maximum firmware size is (2*) 6976 KB, 6144.3 KB is the current size of the OpenWrt firmware image.

Does that mean whatever is shown by df -h does not include the space occupied by the firmware? I'm not too familiar with the command.

The kernel is not part of the rootfs and therefore not known to df, neither are u-boot, u-ubootenv, sysenv, etc.