Funny enough, I never wondered about that bit of magic when I first started out but like you, I did have a tough time realizing how little fertilizer is actually in commercial bottles. But here's the thing, that concentration in flourish was meant for non high light, non co2, low growth rate tanks that most of the populace had about a decade ago. A predominantly fish loaded tank with a couple of low demanding plants. And if that's the kind of tank you keep, its recommended dosages might still work. The hobby has advanced much and as such, we now are able to push much harder on plant growth. In addition, you'll see a lot of tanks on here predominantly loaded with plants with just a few fish. As such, our fert demands are an order higher. To economize, as flourish is not cheap, you'll find that most of us buy dry powders (salts) of the nutrients and mix our own. That's sort of predominantly the reason why that nutrient calculator site exists. If you're not ready for that yet, some fellow hobbyists do sell higher dose pre-mixed fertilizers too. Look for nilocg, thrive, apt complete, or aquarium co-op liquid fertilizers and a few other hobby oriented shops have made their own too.
It's very cool how this hobby is constantly evolving and the science and behind it changing and updating. I'll just have to make sure I'm adding enough if flourish is kind of dated. The thing about the graph is that unless you kind of know how much your plant is uptaking you wont really be sure of your nutrient levels over time. if you get that number off the ppm of a nutrient can vary widely. and you could actually be over dosing or even under dosing. After messing with it a while the larger water change the more forgiving the ppm shift is to an estimated uptake value. Which makes sense.
I guess if i wanted to estimate my NO3 uptake I could dose flourish nitrogen for a certain amount of days, assume the plants uptake none of it and calculate what the ppm of the water would be, then test the water and find the actual ppm. The difference would be my plant uptake.