I've got a NI-9219 module with specs of 100 samples/sec/chnl, but I can sample at 1 million sampes per second no problem. Is that because I'm using a virtual module in MAX and not a physical module?
Try reading the data as a waveform and then inspect the "dt" field. I'll bet DAQmx is automatically coercing to the highest *supported* sample rate without throwing an error. (Though perhaps there's a warning?).
It's a pretty common (but not universal!) behavior for DAQmx to coerce inputs to the nearest legal, supported value.