It's actually due to ISO 9995.
Depictions on the keytops
According to ISO/IEC 9995-1, the level is indicated by the row where the character is depicted on the keytop:
* Level 2 (“shifted”) above of Level 1 (“unshifted”)
* Level 3 (“AltGr”) below Level 1 (“unshifted”).
The group is indicated by the column on the keytop:
* The first or “primary group” at the left keytop border
* The second or “secondary group” at the right keytop border
Additional groups (if existing) in between.
When letters on a case pair are associated with a key, only the capital character need to be shown on the keytop for the primary group, while the lowercase character only is shown for the secondary group.
ISO/IEC 9995-3:2010 applied to the US keyboard layout