if you measure around 2PM, it will likely be a good 5V higher than at 9PM at night, that is the contribution of neighbors solar panels pumping power in locally and the power grid running a bit high in expectation of large industrial loads,
As for the line voltage becoming higher when switching on your pump, if you have a large amount of switch-mode devices without PFC then an inductive load could be improving your power factor and thus lowering the voltage drop to your house,
as its a 317/337 pair the highest ideal center-tap transformer you could get away with is ~26VAC (+-35V DC when rectified), which means for every volt your mains changes, it only changes about 0.2V on the un-rectified AC, which is going to be less than the ripple from your smoothing capacitor unless you use a very big value or drive a very small load,
when building your supply, aim to run it 5V below the rectified mains DC voltage to give yourself some head room, as if the input voltage drops to less than 2V below your desired output it will then drop, (drop-out voltage) if you do this you wont even notice your mains shifting,