So is this the first batch where you used the refractometer ? How do you take your readings? I have a refractometer and hydrometer, and take refractometer readings a couple minutes after reaching a boil because you get a little better mixing (reduction of stratification) of the wort. I place a wort sample in a jar with a lid, into the freezer for a couple minutes before testing. I used to use an open container for samples, but the evaporation while cooling in the freezer gave me inaccurate, slightly higher preboil readings. Also, refractometers can sometimes give erratic readings, so I check my sample 2 or 3 times to be sure I'm getting a pretty consistent reading before feeling confident in it.
EDIT - I also calibrate my refractometer before every use.