LEDs do wear out over time and not all of them are of equal brightness to begin with (especially older ones) so it is possible that this is the case. But for a low power device like this it is also very easy to supply power the wrong way and have some of it reach the LEDs through an unexpected path, causing them to light more dimly than they should.

You can see this problem with cheap powered USB hubs, which do not block their power from travelling upstream to the host. When you power the USB hub and the computer is switched off, a little of the power travels back up the USB cable and partially turns on the motherboard, causing the power LED to partially to light up. Many people have problems with this in that they cannot switch their computer on when it is in this state, because it already thinks it is powered up due to the power leaking in through the USB ports.

wow this is exactly what happens on my Android tv box with a portable hard disk plugged on usb (hard drive light is on when Android box is off) , I never knew why, and it's exactly like you say it won't turn on when in this state
totally off the topic but learn something new thanx

But that's a different matter, power is not coming from those pins they just select the configuration.
May be i should try with a different jumper tough.

Yes but if the pins are connected incorrectly they can accidentally reroute power in a way that the manufacturer didn't intend, which could be why they are only dimly lit.

If you can get three cheap AA batteries to generate 4.5V and use that to power the board instead of the PC power supply, then you can go mad with the jumpers because if you accidentally short circuit it for a moment the batteries can't deliver enough current to damage anything. If you are worried about damaging it by experimenting then this is a way to make it quite safe.

Finally i tried shorting TL and L and the display was stuck at 10. So i left it between TL and H.