The power calc is an example. Not measured.
Your measurement is right. But I assume it's without signal. Now apply a hefty audio signal and see it go up.
And then there's the digital part. Or, like on my Sound Devices USB Pre, metering... N LED's at a few milliamps each...
The point is that 5V 500 mA isn't plenty if you are the design engineer. So you need to cut some corners. It's also why the cheaper Behri's don't have a mic preamp, or phantom power. And the Behri's that do have those features, are externally powered. Behringer wants to avoid returns and support.
Lots of computers are quite happy delivering more than 500 mA on the USB port. The old Apple G4 Cube, fi had USB ports that supplied up to 2,5 amps each. Needed to power the external amp/speakers. That amp/speaker thingy doesn't work on any other computer, unless you open it up to mod it to external power.
Also, even 24 bit devices can suffer from the same whine. Take one of these newer 5$ Chinese USB key audio interfaces, fi. But these are rare. Most of those are only 16 bits.
It happens a lot less with good gear. But it's hard to tell sometimes. The Yeti USB mic, fi, often whines on some computers. But Blue Mics also has a much more expensive Pro model that doesn't seem to have the problem. Unfortunately the have five or six USB mics with almost the same name.
The point is: the whine comes from the computer, not from the connected gear.