Refractometer vs. Hydrometer in Wine Making

Here is an issue a customer is facing. Any help is appreciated.

I'm a small scale wine-making hobbyist. I've made about twenty 1 gallon batches (16 different varieties) of wine since June of 2008. I was led to believe that measuring the specific gravity with a refractometer would be more accurate than using a hydrometer to track the fermentation process. After about 6 to 8 months of racking, I take a final reading at the time of bottling. My problem is that the refractometer reading has never dropped below about 6.5% to 7.0% Brix (scale is 0-32% Brix) during the fermentation process. I start at about 22.5% to 23.0% Brix. The wine tastes dry at bottling time. A friend finally talked me into using a hydrometer to double check the refractometer readings. I opened a bottle and it read 7.0% on the refractometer (around 1.025 S.G.). The hydrometer floated at 0.998 S.G. The refractometer reads zero with RO/DI water. Any ideas what could be causing consistent erroneous readings in this process? I don't believe the refractometer is defective. I don't believe I'm operating the refractometer incorrectly. But, I can't come up with an explanation.

A refractometer can be used to determine the starting sugar concentration, but cannot be used during the fermentation without some mathematical manipulation. The link below gives the conversion.

http://valleyvintner.com/Refrac_Hydro/Refract_Hydro.htm