Why do I need more vcore when I up the fsb?

I have an 8RDA+. Why is it that my cpu will do 2079 with 180fsb at 1.65v but to go to 200fsb and 2000 I need 1.8v. Does this sound normal or is there a problem? My +5rail is at 4.89 on my 420 watt channel well power supply. Thanks!!!

gplracer,
I also have the same problem.
With 1.7V for CPU, it does 2.3GHz at 200FSB (11.5*200). Just finished 15hrs+ of Prime95.
However, it fails to run 200FSB at 2.1GHz (10.5*200) even if I have the voltage at 1.85V, won't even enter Win2k SP3.
Not sure what's the problem but who cares, it runs 200FSB flawlessly at 2.3GHz now.

Here are some stats:
180x11=1997 prime stable at 1.5v
180x12=2167 prime stable at 1.65v
200x9.5=1900 prime stable but 1.75v needed
200x10.5=2205 no go overclocking error stated failed for me too!
210x9.5=1995 prime stable but 1.75 needed
220x0.5=2090 prime stable but 1.8v needed

According to mbm the lows are as follows:
+3.3 @ 2.75
+5.5 @ 4.89
+12 @ 12.02