I can’t comment on V light curves, but I am always wary of visual estimates of the magnitude of Megrez. The comparison stars typically used (alpha Dra, chi UMa, psi UMa, lambda Dra) are quite a distance away on the sky from Megrez and can for much of the year be at significantly different altitudes and thus affected differently by any haze present. This inevitably affects the reliability of visual estimates made under such conditions and increases the visual light curve scatter. (This issue also affects other ‘isolated’ stars such as epsilon Peg, for which there have been claims of large brightness changes over the years). It is also the case that visual magnitudes made when the star or its comparisons are close to the limit of visibility show more scatter than when the stars involved are easily seen.