point grey fl3-ge : image acquisition timer intervals

Hi, I have a point grey fl3-ge gige ccd camera. I configure the TimerFcn callback for triggering 1 frame at every 50ms. The shutter exposure is 20ms. After retrieving the frames, I see a non fixed time interval between successive frames as shown in the histogram bellow. Its centered mainly at .0435 and there is a secondary peak at .0870.

Why its not 100% centered at 0.05? Any ideas?

Thanks, Gabriel

%%ccd initialization : pointgrey  fl3-ge-13s2m-c

vid = videoinput('pointgrey', 1, 'F7_Mono12_1288x964_Mode0');
src = getselectedsource(vid);

% ccd vertical window
vid.ROIPosition = [0 382 1288 200];

% trigger config
triggerconfig(vid, 'manual');
vid.TriggerRepeat = 100-1;
vid.FramesPerTrigger = 1;

% timer config
vid.TimerPeriod=0.05;
vid.TimerFcn = @trigger;

% frame processing
%vid.FramesAcquiredFcnCount  = 1;
%vid.FramesAcquiredFcn = @frame_callback;

src.ShutterMode = 'Manual';
src.Shutter =20;
src.FrameRateMode = 'Manual';
src.FrameRate = 20;

src.Brightness = 0;
src.ExposureMode = 'Off';
src.GainMode = 'Manual';
src.Gain = 0;
src.GammaMode = 'Off';
src.SharpnessMode = 'Off';

src.PacketDelay = 400;
src.PacketSize = 1400;

%%acquisition starts here

start(vid);

[ccd_data, ts] = getdata(vid,100);
i=1:100-1;
tdifs=ts(i+1)-ts(i);
hist(tdifs)