I have a lighted moose outside, and on one side the bulbs look like they are running at about 10% power, There are a few within these lights , that are fully lit, I have tested them all out, replaced the bad ones and replaced all of the little fuses.
IF you replace some of the light bulbs and the new ones don't precisely match the old ones in terms of current draw (not easy to predict in advance) then some will be brighter than others. Usually the ones that are brighter will tend to burn out more quickly compared with having perfectly matched bulbs to begin with.
Do you get coverage you can count on? From the bag of fertilizer? From your lawn sprinklers?