The difference is due to both AS SSD and the SandForce controller in the 520.
AS SSD uses only incompressible data for its test. The SandForce controller is designed to work at its best with compressible data, and does not perform as well with incompressible data, reducing its write performance. Real world data is ~75% compressible, and the AS SSD results for a SandForce based SSD will always be lower than non-SandForce SSDs.
So AS SSD is both a synthetic benchmark (its operation is not what happens in real world use) and it uses data that is not representative of real world data. It also places a higher value (more points) on high queue depth performance (4K-64 Thrd) which is never encountered by a SSD used in a PC. Some SSD manufactures have tuned their firmware to provide high queue depth performance to produce impressive looking numbers in AS SSD.
AS SSD is useful in several ways for verifying a SSD's performance, but as a single, overall judge for comparing SSD performance in actual use, it is far from perfect.