Just too different measuring scale. Just like Celcius and Farenheight for temperature. THere are numerous conversion scales out there. You can down load calculators or charts. Almost every brewing book I have has a chart in it as well.
Here is a text clip from our sister site Homebrew Talk's Wiki Page on the subject.
Brix can be approximately converted to specific gravity (SG) by a simple equation:
SG = 1 +(0.004 x Brix)
Most people just remember the multiply by four rule. Take the Brix reading, multiply by 4 and this will give you specific gravity in “gravity points.” For example, if you read 11 Brix, multiplying that by 4 yields 44, which corresponds to a specific gravity of 1.044.
A slightly more accurate conversion formula is:
SG = 1.000019 + [0.003865613(Brix) + 0.00001296425(Brix) + 0.00000005701128(Brix)]
Using this formula, a Brix reading of 11 yields a specific gravity of 1.043.