The monochromatic RMS OPD is referenced to the chief  ray or centroid of the wavelength you are tracing. But for the polychromatic case, you need a common reference. This means in general the poly RMS is larger than the average of the mono RMS values.
 
Here is an easy way to visualize what's going on. Imagine a diffraction grating followed by a perfect lens that splits say 3 wavelengths into 3 separate perfect Airy disks. The RMS OPD for each of these wavelengths individually is zero, but since they don't all land at the same spot the RMS OPD is large if you pick a single reference point for all 3.
 
Another way to visualize this is to consider MTF. Let's say you have excellent monochromatic modulation, say 90% at each of three wavelengths individually. But if you have lateral color, the peaks and valleys of each wavelength are offset from each other. The polychromatic intensity modulation gets reduced since it never goes as low as the individual mono intensities do. So you may get a much lower modulation, like 20% polychromatically.
 
All of these effects occur in the presence of lateral color.