memory speed question

I have a host clock speed of 133Mhz. It is applied to an Athlonxp2000+ chip. I have PC2100 DDR RAM. This means that the effective frequency between the CPU and Northbridge, and between the Northbridge and RAM is 266MHz.
Under this assumption, the memory transfer rate should be 8bytes*the effective frequency(266MHz) = 2128 MegaBytes per second. I ran a diagnostic program and it said the memory throughput was only about 675MegaBytes per second. Why?

 

What memory tester are you using? with AIDA32 I get about 5.2GB/s or so, when the technical throughput is 6.4GB/s .. so there is a difference between the technical throughput and the actual throughput

also keep in mind that if your FSB is 133MHz it's bottlenecking the speed of your RAM .. 8 * 133 = 1064 .. so there you go

 

I was under the impression that with the double pumped amd motherbaords, that the base 133 was applied between the cpu and north bridge, and it automatically sent data on the leading and trailing edges of the clock pulse. The same 133 is applied between the northbridge and ram, and if you have ram capable of the ddr then the spd chip on the ram module tells the northbridge to send data twice per clock tick instead on just once. But if you installed single data rate ram, the spd would tell the memory controller on the northbridge to send data only once per clock tick. The only tangable way I came up with to test whether the ddr is operating like it should was to run a memory bandwidth test, the program I used is called Dacris Benchmark 5.0.
What are your thoughts on this?