While the first part of this statement is true, the second is where the problem lies.
The reason for the first statement to be correct is not how much you produce, but many different variables that have nothing to do with that. Like how much convertion each individual will have from aromatize and 5 alpha reductase enzymes into estrogen and DHT for example.
Also, thyroid fuction can interfere with absorption of creams in certain individuals and whatnot, making them non-responders to that method.
Like i said... there are way more variables, but these are some, just to name a few.
The reason a doctor tests you to adjust the dosage is to keep in the normal range through increasing or decreasing the dose according to what your lab results indicate, as too low or too high can cause unwanted side-effects.
However... the reason why the dose was too low or too high, nobody can usually pinpoint... and it isnt the result of the individual endogenous production like you seem to think