Read speed: up to 550 MB/sec
Write speed: up to 515MB/sec
what your seeing is that your drive supports up to 6gb/s total bandwidth, but the drives don't actually use it all, there is some overhead which isn't being used, now while that may sound and you may feel cheated, to actually get the full amount of bandwidth would be nigh impossible, the next step would be to use hsdl (high speed data link) drives instead which surpass current sata 3 restrictions up to 20gb compared to sata 3's 6gb.
what to remember here is that everything is UP TO a given figuire they in no way state you will get those figuires all the time or ever, now i'm guessing they are average figuires over a set period of time and may go up or down, but true full sata 6gb is probably not actually possible all they can ever do is get close while using that interface.
it's all in the wording really, granted your not entirely wrong to feel cheated and they should be clearer about what your actually getting, but all they state is that they use the sata 3 interface as it supports bandwidths up to 6gb where as on a sata 2 interface you would be capped on the same drive to that interface's maximum, and you may not reach the full amount either.
so for future reference, the drives read and write speeds are what dictate how fast the drive actually is, yours is listed at the top of this post, other things that affect ssd drives are mlc or slc, single or multi layer cell, single layer cells run incredibly fast and are more stable, yet multi layer cells can store more data, current mlc drives are slightly slower than their single cell counterparts but not by much and they are improving through upgrades in the controllers on board, intels latest drives are reportedly very good mlc drives for example with a new controller, the controller ( a small chip that does all the interfacing) is arguably the most important component on an ssd a poor one will mean poor performance.