Because any event causes al_wait_for_event to return, meaning offset is increased every time you press or release a key.

If you print something every time you get a KEY_UP, KEY_DOWN, or KEY_CHAR event, you will see what happens.

Basically, in any game, what I do is this:

int keys[256]; // global, or somewhere else

// in allegro_key_down of events
keys[event.keyboard.keycode] = true;

// in allegro_key_up of events
keys[event.keyboard.keycode] = false;

Then, every "frame" of the game, I have a logic(); and draw(); function for every object. All objects.logic() are called, then all objects.draw() are called.

In logic(), an object will check if a key is down that it cares about:

if(keys[ALLEGRO_KEY_DOWN]) // down arrow
{
// move guy downward
position_y += 20;
}

then finally the draw() code gets called.

We've "decoupled" a bunch of unrelated things. Logic and drawing are separate. And object logic is separate from the act of handling whether or not a key has been pressed (controller logic). Objects don't respond to keyboard events, they only check whether a specific flag has been set.

That way, objects can never react to the same event twice by accident (otherwise, if you press a key faster, your object can go faster in a frame).

Instead of directly checking ALLEGRO_KEY_DOWN, you can also have generic actionUp, actionDown, actionLeft, actionRight functions in an object. Then those functions call whatever "key" they are pointed to. So now you can have two or more objects, and they check their "up" key respectively without having to write more code.