You don't state how you are measuring the gravity. If you were measuring hot wort and using a calculator to adjust the reading to the calibration temperature of the hydrometer, then you may have an error based upon the actual wort temperature versus what you used for the adjustment. Depending upon your system, if you measured the gravity of the wort and then started recirculating wort which was in pump hoses, this volume may have made a difference.
It could maybe be that the dextrose was not fully dissolved at your second reading, though that would not explain why it would be lower than your initial reading.
Either way, I would measure you post boil gravity and the use a gravity point balance to help determine which reading might be in error.