I take samples with my refractometer from the top of the mash and find that the gravity is lower than expected which makes me worry. If I then stir the mash the gravity goes up. I think it is because the sugar laden wort settles to the bottom of the tun because when I don't stir I still get the expected OG at the end of the boil. If the sugar laden wort really settles then at the bottom of the grain the sugar concentration is higher which would lead to poorer efficiency if I didn't sparge. Stirring equalizes the concentration so a little more of the sugars are dissolved. With sparging the won't be much difference at the end.
Do you fly or batch sparge?