Because there’s no rule or/and law that tells you the exact percentage.
Many people posted this as answer already, but I want to make that clear one more time, since I feel some people especially beginners are looking for a exact number.
This isn’t math were you have only one formula and one correct end result. In fact many roads lead to rome, some get there faster or slower then others, but the goal at the end is reaching rome.
Based on mixing it’s the same and we can’t stress enough single flavor testing. Start low and work up, mark the first time you taste the flavor till you taste something off/weird/muted/harsh etc. (Thanks to Smoky for that tip lol) write your notes and that’s your percentage and median. Or in other words “welcome to rome”
If you wanted to go by a median, that’s created by others based on their taste buds, look at the median and subtract 1-2% (depending on brand) and start there.
But again you wouldn’t know if the flavor is strong without testing it yourself at multiple percentage, steep times and hardware.
That also answers your prior question. Some stores do it, others dont. Some use 1 flavor and tons of sweetener, others dont. ETc. Can’t really answer this with “yes/no they all do/dont” kind of thing.