Simple Period Measurement

Hopefully this will be an easy one for somebody, but working on site under pressure and I can't see what is wrong

I have a PCI-6351 card and I am testing it with NI-MAX, I setup a simple task to measure the period of the TTL level square wave running at 500Hz.  I don't put much faith in the function generator for the frequency value but the signal is clean.  Using NI-MAX I run the task and would expect to get constant values around 0.002 seconds, but I get all sorts of values.  The image below shows the results and settings.  

There shouldn't be anything more to it should there?

Thanks much,

Mande.