Water adjustment question?

I've been searching a bit for this and can't find anything on it I'm hoping someone here can clarify.

When adjusting your water I've read that it is recommended that you split your additions by volume and I can't figure out why. I would think that it would be best to put all additions in the mash water as that water would be incontact with the additions the longest. Also since it is later combined with the fly/sparge water in the boil kettle why would this matter?

I feel I'm not asking this the best but can someone point me in the right direction here?

I line up my gallon jugs with my mixed water and add the salts to each gallon jug equally. It's just how I've always done it. A bit more inconvenient at first, but later on I don't have to worry which water goes where.

My ph stays in range for two batch sparges, so this method works well for me.

If you added all the additions to the mash, you'd be screwing up the pH. If you needed to add like 6 grams of gypsum was right for say 8 gallons of brew water, you wouldn't want all 6 grams in just the 4 gallons of strike because it's twice as concentrated. Of course, maybe that's exactly what you want. It's just more complicated to figure out.