Thor I'm using tuning_color_low_light.bin
In that case it's hard to say what the reason is. Might be coded in a way that allows max ISO to be 2800, but is clamped so the value can't reach it's maximum due to (often) stability issues or other hardware constraints. Wouldn't know for sure, but the functionality should still be the same, no?
Are you experiencing any issues with AE on the custom blob?
Thanks,
Jaka