You'd think that you can run your 100MHz SDRAM at 100MHz FSB, right?
Alas, this is not the case. The reason is that with perfect timing, a manufacturer can insure that all signals arrive at exactly the same time at the pins on the chip. Then the chip can be made to run at 100MHz. (10 ns cycle time)
On a real motherboard however, the trace lengths between the memory controller and the RAM are not all exactly the same length. Maybe there are imperfections in the pin-drivers on the memory controller. The result is that the signals from the memory controller will reach the SDRAM chips within a small timeframe. For example: 2ns. The minimum cycle time then becomes about 12ns, leading to a max frequency of 83MHz.
This is similar to the harddisk manufacturers using gigabytes of
10^9 instead of 2^30. The bigger the number, the more likely
they will be chosen over their competitors.
This also holds in the CPU world. That's why a Cyrix PR166 processor only runs at 133MHz. They needed the higher number just to compete with the Intel parts that did run at 166MHz, although (cyrix claims) the Cyrix processor could perform 25% more work in the same number of cycles.