I try to understang the event: "onEnterFrame"

[tiptoed]

Hi everyone,
Apparently I dont understand the event named: "onEnterFrame" and I wondered if someone here would care to explain that to me.
I assigned the following code to a 1 frame clip with a 1 movie clip named: "movieClip":

movieClip.onEnterFrame = function() 
{
i++
trace ("onEnterFrame called"+i);
};
var i:Number=0;

At the Output panel appeared hundreds of numbers as if the clip was consisted of hunderds of frames.
As I understand it, the event "onEnterFrame" is activated when a clip changes frames.
There is only 1 frame at that clip, how could it show 500 times an event of changing frame?
Thanks