The metabolite is the remnant of the steroid compound, e.g., nandrolone, boldenone, etc., not the ester. The ester is simply hydrolzed by esterase enzymes. The steroid compound cannot be used until the ester is cleaved away from the compound so that the steroid will attach to the receptor cell. This is when the body begins to metabolize the steroid. The ester is irrelevant at this point. There is no difference between endo and exo test from here on, and that's why there's a range, not a positive or negative as in nandrolone, stanozolol, etc. (synthetic androgens).
Interestingly, it has recently been proven that nandrolone can be produced endogenously by humans. And this has screwed up the whole IOC, et. al. testing. From what I've read, the official stance has been to change nandrolone from a (+) or (-) test to an acceptable quantity test, similar to testosterone.