If you use it fresh, it hasn't had time to drop pH. That's my take on it.
I have a pH meter, but I've never done any measurements on Starsan. I usually mix 1/4 ounce iwth 3 quarts of water, so it's extra strong. It's just easier for me since the measuring thing starts at 1/4 ounce and my Walmart water jug is 3 quarts.
I can fill up a spray bottle several times with that jug, and I also pour from the jug into a keg or other large vessel, sanitize it, then pour it back into the jug.
I can go MONTHS on that 1/4 ounce.