Hello,
A possible reason for this difference might be the temperature.
Please be aware that the maximum error over 0 °C to 85 °C is 1.5% in the MPXx6400A device.
Are you exposing your application to temperature changes?
In order to get more information about the temperature error, please refer to the figure bellow:
I hope this information will be useful for you.
If I misunderstood your question, feel free to let me know. I will be glad to help.
Have a great day.
David Diaz.