When you see a dac with a USB limited to 16/48, this is a generic USB. Other Dacs have 24/96 USB (Benchmark, Bel Canto) is better. The absolute best is "Asynchronous" 24/192 USB (HiFace, Ayre, Wavelength).
The difference is the clock and timing.
With the car engine analogy, Early cars had it's engine spark timing mechanically linked (slaved) to the engine (the clock) - this was inferior (generic USB), because mechanical wear and tear affect the timing of the spark - the computer affects the timing of the audio data whenever the hard-drive kicks in, a software process is processing, wireless activity - etc.
Modern cars are different. A computer reads the bits output of sensors that read the engine speed - the computer sends its own timing of spark programmed into it - this program times the spark to the engine - regardless of engine wear. The spark timing is no longer "slaved" to the engine. The clock is no longer the engine - it is a dedicated electronic component - dedicated to timing. This is Asynchronous data transfer - or Async USB - superior timing.