GRIB2 load for GDT90 (space view).

I can offer two arguments:
 1. `Nr` is an integer, so if the altitude-ratio was given by `section['Nr'] * 1e6` then we would only be able to specify the altitude in jumps of one million times the Earth's radius!
 2. The `section['Nr'] / 1e6` interpretation is consistent with real-world GRIB2 messages which have values such as 6610674. In other words, the satellite altitude is 6.610674 times the Earth radius.

More prosaically, it might help when reading the GRIB2 manual if you imagine an additional comma after the word "radius":
> measured in units of the Earth’s (equatorial) radius, multiplied by a scale factor of 10^6

I.e. Take the ratio to the Earth's radius, and then multiply by one million.