Hey Brady,
A couple things here: looking at the USB-6009 data sheet it looks like, for differential signals you would have 14-bit resolution, so your original numbers may be correct.
As for the quantization step, those calculations should be done using the range set for the DAQ task, posting your code could be helpful here. It seems like the closest match would be that you have it set up as a differential task on the +-20V scale, which would mean 16384 steps over 40V, or 2.44 mV. While not the 2.54 you are seeing, it is close enough. So you could optimize your measurement by moving to the +-5V scale. More info here.
Now with all that said, quantization steps isn't a good measure of the accuracy you can expect as it doesn't account for various forms of errors that will be in the measurement. Take a look at the Absolute Accuracy section of the spec documents to figure out what you can expect in terms of PSI reading resolution.
Sam J.
Applications Engineer
National Instruments