I think you got the calculation I used correct:

"448 km - 280 km = 168 km of range used

You then multiplied by 150 wh/km to get:

168 km x .15 kWh/km = 25.2 kwh."

The rated range is always based on 150wh/km, that does not fluctuate. Using this, you can measure the amount of energy in the battery at any given time, using assumption of 75kw at 100%, 67.5kw at 90% (my full charging status), and 42kw at 56% or 280 km of rated range. To go from 42kw back to 67.5, that's 25.5kw of energy. There is probably a little variation in those numbers since I don't know how many kw are left at 0km, but that's another topic

The car "claims" to have consumed 19kw, so my question is, what happened to the other 6.5kw of power?

The error in your calculation is assuming that the efficiency remains constant at 150 wh/km and does not fluctuate. Any car, even an ICE will have different efficiency depending on driving style, speed, etc. This is just an average, and depends on many factors such as how fast you drive, the outside ambient temp/weather conditions, elevation changes, if you are using heat/AC.

Around this time of year you should be getting around 180-200 wh/km, so travelling 99km you would consume between 17.8 - 19.8 kWh, which is exactly what you consumed.

In your centre console you can track what your consumption is via the trip meter (also v9 includes an energy display which has more info on it, but the simple trip meter is also good enough). For example, here's a shot from my car last week when I went to the gym in the morning, which is a short drive, but also slightly uphill from my house and since the temps are a bit cool the efficiency was off of my average of around 155 wh/km:

If I used the whole battery at this rate, I would get 360km to a full charge.

But a few days before I did a drive which was on average downhill (maybe a few hundred feet elevation drop), and my efficiency was 2x better:

If I used the whole battery at this rate I would get 757km to a full charge....but since this trip was downhill sooner or later I would have to go up a hill and the efficiency would average out.

The efficiency is really just a physics calculation...you have a certain amount of mass, and it takes energy to move it. The faster you move it (highway vs city driving), or if you are moving with or against gravity (uphill/downhill), or into/with the wind all affect the final number. Regardless of Tesla, Volt, Bolt, Leaf, Hummer H2...all have the same physical limitations.

If you take any gas car an 'punch it' at every light vs trying to hypermile you will see a 2x difference there as well.

Hopefully this helps clear things up...just wait until it gets really cold, you'll probably be around 220 wh/km or worse depending on how much you heat the car