What causes Voltage fluctuation in Panel
It's fluctuating within spec, because your household (and neighborhood) electric system is real, not "ideal."

The "240V" feed to my building varies between 238 and 242 on a regular basis - this depends on what other loads the system is servicing and the resistance of real wires with resistance. Actually, it's pretty impressive that the variation is that small, when you consider the distances involved and the number of separate, uncoordinated loads the power company has to supply, as they turn on, or off, at random.

Within your house, you may see variation on the two (115-125V) legs due to more loads on one leg than the other (unbalanced) - but the overall voltage into the building also varies slightly.