As the water sits there it is going to continue to balance out and if you run an air stone it can change even more.
I find that with phosphoric acid pH down, like that from General Hydroponic, the pH will raise as much as 0.6 in a 12 hour period. By allowing the solution to rest and balance you get the maximum effect of the chemistry when you use it.
Nitric Acid pH down, TechnaFlora, holds a much truer and more consistent pH level over a 12 hour period and as a result you will use less of it.
Remember that each of these products adds more elemental salts to the solution. I've used as much as 35ppm (about 3ml per gallon) of phosphoric acid before to get it to stay at 5.8.
Something I have read recently is that with A+B style nutrients you want to mix them in 1/2 the water at twice the strength.
So if you had a 10 gallon reservoir, you would use 5 gallons of water, and 10 gallons worth of A+B. The decrease in volume of water allows the combination of the two products to complete quickly. In a larger reservoir when using maybe 3ml of A and B it is "hard for the parts of the two to find each other." This problem is compounded when products like Cal-Mag or other fertilizers are used right afterwards.
Mix up the first two nutrients in half the water. Let it rest for 1/2 an hour. Then add the rest of the water, the rest of the nutrients, and let it rest another half hour. Come back, pH a balance, and let rest one more hour. Balance again. The solution is fine to use now.
Always check your pH before using any solution. Don't depend on it to stay perfect. You need to stay vigilant.