wvtool frequency domain units off by factor of 2

The documentation for the wvtool states "By default, the frequency domain plot is the magnitude squared of the Fourier transform of the window vector in decibels (dB)." Examples show that wvtool appears to plot the 10*log10() of the magnitude, not the magnitude squared. For example, for a simple rectangular window which is well known to have the first sidelobe at ~13 dB the wvtool shows it only at -6.6 dB.

I am using R2108b.