In order to determine the power of a card, you cannot look solely at the clocks. The clocks tell yo how fast each core int he graphics card is running, so if a card had 100 graphics cores at 800 mhz, it would be more powerful than a card with 50 cores at 600 mhz. In this case, the 550ti has 192 cores running at 900mhz, versus the 460 with 336 cores running at 810mhz. Without taking the shaders and other factors into account you can calculate the relative core power by multiplying the # of cores, by the core clock.
460: 336 x 810 = 272160
550ti: 192 x 900 = 172800
(This is not an exact nor precise representation of power, it is simple a visual aid)
That explains why the 460 is more powerful, if you have any more questions, I would be happy to answer them