This usually happens with a periodic scan of key presses. This speed often is limited by the mechanical key response time, the protocol to transfer those key presses (here USB HID), and by the software it is used in.
The [HID specification](http://www.usb.org/developers/hidpage/Hut1_12v2.pdf) tells what a keyboard can actually send through USB to have a chance to be properly recognised. This includes a pre-defined list of scancodes which are simple numbers from `0x00` to `0xE7`. The firmware assigns a scancode to each key of the keyboard.
As the layout is generally fixed (unless you create your own), the firmware can actually call a keycode by its layout name directly to ease things for you. This is exactly what is done here with `KC_A` actually representing `0x04` in QWERTY. The full list can be found in `keycode.txt`.
Putting aside shortcuts, having a limited set of keycodes mapped to a limited layout means that **the list of characters you can assign to a given key only is the ones present in the layout**.
For example, this means that if you have a QWERTY US layout, and you want to assign 1 key to produce `€` (euro currency symbol), you are unable to do so, because the QWERTY US layout does not have such mapping. You could fix that by using a QWERTY UK layout, or a QWERTY US International.
You may wonder why a keyboard layout containing all of Unicode is not devised then? The limited number of keycode available through USB simply disallow such a thing.
You can have the firmware send *sequences of keys* to use the [software Unicode Input Method](https://en.wikipedia.org/wiki/Unicode_input#Hexadecimal_code_input) of the target operating system, thus effectively entering characters independently of the layout defined in the OS.
