Cam, there's a small chip on all DIMMs these days called the SPD (Serial Presence Device), it contains the timing information for that stick of RAM. When the BIOS is set to Auto it reads the SPD and sets the timing to what the chip specifies.
If you have a DIMM which you think should be clocked higher than your BIOS is setting it, it may be that the SPD is misprogrammed. That has been known to happen with cheaper memory manufacturers. In that case, if the RAM is actually stable at the higher clocking, one can go into the BIOS and bump the speed up manually. But a lot of times the cheaper RAM with the misprogrammed SPDs isn't stable at the stated speed (so it may be that the SPD isn't wrong after all).
