To name a few....
Unlike smartphones FiiO and others (like my Eversolo) develop their own customized Android Audio core which bypasses Android's restrictive and sound degrading internal sample rate conversion.
Android phones don't have dual high precision low-noise audio clocks.
Android phones don't use the flagship AKM DAC combo chipset AK4191/ AK4499EX for complete separation of digital and analog circuitry. My SMSL DAC and Eversolo Streamer / DAC / Preamp both use this unique AKM chipset as does your FiiO.
The phones don't have the latest generation XMOS USB interface for maximum compatibility and to support higher-res audio including 768KHz/32bit PCM and native DSD 512.
I could probably go on and on but these stand out off the top of my head.
