Refractometer calc question

Brad - BeerSmith gives the following SG conversions of a Brix reading.  The other SG is from the complicated formula found everywhere online, which better mimics the standard “four times Brix, up to 12º” shortcut we often hear.  BeerSmith appears to be three (3) SG points low at a Brix of 18º.  Other online tools seem to match the formula that gives higher SG numbers.  http://www.onebeer.net/refractometer.shtml

Brixº Bsmith SG Formula SG
10 1.039           1.0409
12 1.047           1.0492
16 1.064           1.0663
18 1.072           1.0750
20 1.081           1.0839
22 1.09           1.0928
24 1.099           1.1019

The problem is that the "four times brix" number is associated with a brix hydrometer, and not a refractometer.  Most refractometers are calibrated to measure pure sugar, and not complex sugars like maltose.  That is why you need to go through the calibration process with BeerSmith and use the refractometer tool.

I'll try to pull the actual formula used in BeerSmith - I don't recall what it is off the top of my head.

Cheers,
Brad

http://www.primetab.com/formulas.html

Thanks.  Here's the one I mentioned, written out long-hand.  It's toward bottom of page.  It does mention Table 88 Sucrose.

I have new batch of ale that has been fermenting 10 days.  The starting Brix is 18.6/ OG conversion to 1.74.  Today I take a reading of 7.8 Brix. 

I plug this data into BeerSmith - into its calculation for 'Fermenting Wort Gravity" and the reading I get for 'Corrected Specific Gravity': 1.005
If the calculation is made for 'Unfermented Wort' the calculation is for straight Brix to SG conversion: 1.031

I don't know what I'm reading - Corrected Specific Gravity': 1.005
and so far from my expectation that I'm at a loss.

Any explanation that fits these numbers?