O seu SSD usa um controlador SandForce SF-2200, que só obtém o alto desempenho de gravação usando dados que podem ser compactados corretamente. No uso normal, os valores são muito mais baixos (como você pode ver).
SandForce controllers also use a proprietary compression system to minimize the amount of data actually written to non-volatile memory (the "write amplification") which increases speed and lifetime for most data (known as "DuraWrite"). SandForce claims to have reduced write amplification to 0.5 on a typical workload. As a byproduct, data that cannot readily be compressed (for example random data, encrypted files or partitions, compressed files, or many common audio and video file formats) is slower to write.