"The Strehl Ratio is another statistic measure of optical performance calculated from the interferometric test data. It is the ratio of intensity of an aberrated wavefront to an unaberrrated wavefront. It describes the amount of intensity reduction due to wavefront errors. A Strehl Ratio of 0.8 or higher is considered to be diffraction limited. A Strehl Ratio of 0.8 corresponds to an RMS Wavefront Error of 0.0712 or 1/14 ."

How convenient!  A Peak to Valley Wavefront Error of 1/4 is just good enough to be diffraction limited, yet an RMS Wavefront Error of 1/14 is also considered to be diffraction limited. The average consumer isn't expected to know the difference between P-V measurements and RMS values, but does know that a wavefront error of 1/14 is smaller than a wavefront error of 1/4.  The two values represent an error of the same magnitude, but the consumer will likely think the smaller fraction is better.