my UNO's integrated LED stays turned ON..


I've used Leonardo board for about 3 months.

I needed another new board for bluetooth system communicating each other, ordered UNO instead Leonardo.

with three months experience, it is awkward that UNO's integrated LED(yellow) stay "ON" - of course when i use
digitalWrite(13, LOW), LED goes "OFF"

i compare them for several times with simplest example below :
void setup(){} void loop(){}

Here is the result :

  1. when connect to usb(or external power source)
    [Leonardo] : ON
    [UNO] : ON

  2. a few seconds after push reset button
    [Leonardo] : OFF
    [UNO] : ON

I really don't understand why two boards act differently.
It doesn't affect my project, but it annoys.

is there anyone explain this?