pulse train generator - wrong freq output

I am trying to get 50 Hz digital output from a USB-6211 using the code below. The TTL is supposed to activate a device which is able to show a readout of the incoming input (in Hz). The problem is that as soon as I set ch1.Frequency around 40 the device reads half of it (20).

I thought it could be a duty cycle issue. But changing the length of the d.c. does not eliminate the problem. I can "climb" up to 39 Hz...but then the 40 and anyhting beyond this gets halved or reduced to max 30 or 33..

any idea why ?

daq.reset
d = daq.getDevices;
s = daq.createSession('ni');
s.Rate = 20000;   % sample rate
Fs = s.Rate;
s.DurationInSeconds = 5; % video duration in seconds
duration = s.DurationInSeconds;
ch3=addAnalogInputChannel(s,'Dev1','ai7','Voltage'); % this is the camera strobe channel
ch3.TerminalConfig = 'Differential';


ch1 = addCounterOutputChannel(s,'Dev1','ctr0','PulseGeneration'); % camera trigger 
ch1.Frequency = 50; % this is going to be the camera fps
ch1.DutyCycle = 0.2; % amount of time the pulse stays active (from 0 to 1)
ch1.Terminal

ch2=addCounterInputChannel(s,'Dev1','ctr1','EdgeCount');
ch2.Terminal

 lh = s.addlistener('DataAvailable', @(src, event) logData(src, event, fid1));
   lh1 = s.addlistener('DataAvailable', @(src,event) plot(event.TimeStamps, event.Data));
   
   
    s.IsContinuous = true;
    s.startBackground;
     pause(duration);
    s.stop % end recording