StdDev represents the "standard deviation" of all reported speeds for that CPU MHz. The standard deviation is a statistical figure that can be used to determine the typical range in which all of the reported speeds lie within. Typically when there is a high amount of correlation between all of the reported values, then a high majority of the values will lie within reported the mean spead +/- the standard deviation amount. Values that lie outside of that range have a greater possibility of being anomalous.

StdErr% is simply a different representation of the standard deviation. It is simply the result of the following (stddev / mean * 100). StdErr% is useful for comparing the relative amount of deviation between each of the values within the summary listing pages. Typically, if the StdErr% exceeds 15% or 20%, then there is a high amount of deviation within that group of values, and one or more of the values is anaomalous to a large degree.