Why do keyboard keys have (most) letters in top left corner?

I was just looking at my keyboard (Logitech K200) for the millionth time and realized that the names of each key were "mostly" in the top left. The row of numbers at the top is just left aligned with the shift versions above the regular versions (make sense). Normally I would just call this a bit weird and move on. However, I then noticed that not all of the keys are left aligned! My media keys (sound control, calculator, mail, etc.) have centered pictoral representations. The "Print Screen", and "Pause/Break" keys are all center aligned along with the 6 keys in the "page navigation" block. Oddly enough, "Caps Lock" is left aligned while "Num Lock" and "Scroll Lock" are centered.

I realize that this might be different for various keyboards so I went searching throughout my house for various other keyboards. The HP Chromebook I just bought has most of the key names centered. However, "tab", "caps lock", and left "shift" are left aligned (with "backspace, "enter", and right "shift" right aligned).

An ancient IBM Thinkpad (not Lenovo) I scrounged up appears to be exclusively left aligned. An older Dell Latitude D420 is almost all left aligned except for the arrow keys and "page up/down" keys. A newer Dell Studio XPS 15 is mostly left aligned but with arrow keys and function keys centered.

Why? Is there a historical reason and nobody has bothered changing it (except for Google but I'm not surprised)? Have studies shown that the left corner is easier to see? Intuitively, I would have just stuck the symbols in the center of each key.

Answer

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