The amplitude measurements (stddev) are indicative of how much alpha activity is occurring in the subject. However, the amplitude values for beta do not truly reflect the amount of mental activity occurring with the eyes open. What is a possible explanation for this observation?

The alpha and beta waves are out of sync which impairs the actual capture of the two.

Explanation:

The question above presents a situation in which many activities are happening in the beta frequency, since they can reflect a mental activity that responds to visual stimuli, which are captured by the open eyes. However, alpha waves are disproportionately present. This, in addition to causing a little cancellation of the beta wave, causes the capture of the waves to be impaired and only the one that this dominant is captured with more precision.