Well this is because like Freddy said DDR memory has two things that you can
measure the clock speeds with - effective clock and the real clock. The
effective clock in this case is 533MHz, which is 2 times the real clock
which is 266 Mhz. Since DDR is double data rate, it can send 2 times the
data at once, so although in reality it is only sending 266 Mhz, it's
sending 2 data quantities, which makes it 533MHz when it is really being
used.
That's the same way they take a 200MHz bus and call it a 1000/2000 MHz bus.
Typical AMD marketing that it seems has caught on.
Although, on a serious note, do they really take your on-board system clock
and de-clock it then re-clock it per your multiplier? Why? Are the board
clocks (I have 3) just not able to drive anything on their own? I could
understand that, but why not just put a driver on it (like an op-amp or
something)?