nanoDLP

Are you sure you do not have any other option effecting cure time? such as dynamic cure?

I could not reproduce the issue please, share the debug file.

I got it wrong, image being displayed for 8seconds as intended but shutter opens only for 6 seconds, is it right?

If I understand the issue correctly this time, the problem is some servo motors could require relatively long time to open and close which eat into cure time.
You have two options,
1. Move shutter codes into main gcode box (before and after layer) and send open command before movement started or split movement in two parts and send shutter open in the middle.
2. Get faster servo
3. In my experience small servo motors are not suitable for 3d printer workload so they will burn sooner or later, so it is much more reliable just to open them once for the whole printing session.

Based on your log file.

		2017-04-06 00:38:20.588443 Transfering Data ;M106 S38 ; 20W↵;M106 S50 ; 30W↵;M106 S68 ; 50w↵;M106 S100 ; 66w↵;M106 S130 ; 80w↵M106 S255↵↵
		2017-04-06 00:38:20.599967 Received Data From RAMPS: ok 0↵
		2017-04-06 00:38:20.600962 Done message from RAMPS has been received
		2017-04-06 00:38:20.604466 Received Data From RAMPS: Z_move_comp↵
		2017-04-06 00:38:20.607492 Received Data From RAMPS: Fanspeed:255↵
		2017-04-06 00:38:20.787659 Display layer public/plates/3/50.png
		2017-04-06 00:38:20.788158 Curing for 8 seconds
		2017-04-06 00:38:28.838881 Shutter Close
		2017-04-06 00:38:28.838881 Clear screen
		2017-04-06 00:38:28.839894 Transfering Data M106 S0↵↵

On 38:20.58 shutter open gcode sent to RAMPS.
On 38:20.78 start displaying layer.
On 38:28.83 shutter closed.

I believe NanoDLP sent gcodes to RAMPS on exactly correct time. If your servo is fine, you need to investigate issue on RAMPS side maybe one of these commands cause delay on shutter on RAMPS side.