I don’t have the same model and firmware (I use a FB 7490 and Fritz!OS 6.30) and you didn’t show the complete device list.
Is it possible that you maybe use FRITZ "Powerline" adapters? I have only yet seen the blue arrow symbol on newer Fritz!Box firmwares since they now also support (and show) Powerline adapters.
I’d like to show you my list (as it looks on FRITZ!OS 6.30) but unfortunately I’m currently running a beta of what the firmware will look like in autumn 2015 …
This already shows the Powerline adapters in clear, not using the blue arrow symbols (as Fritz!OS 6.30 does).