In men, there are 2 sources of serum estradiol: 1) aromatization from testosterone an 2) direct production of estrogen in testes and adrenal glands
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5835337/#S2title
Hence, the finding that 0.5mg and 1mg of arimidex lower serum estradiol by the same amount implies that both doses completely wipe out aromatase activity. The reason that serum estradiol is only lowered by ~50% is that the body still maintains the direct production of estrogen, mainly in the testes. In fact, estrogen-suppression will, via the HPTA, increase gonadotropins and with it estrogen production, thereby partially offsetting the effects of aromatase inhibition.
It's important to note that this partial suppression of estradiol will only happen in naturals. If you completely suppress your HPTA axis by injecting AAS, then this will also largely shut down estrogen production in the testes. In that case an AI dose such as 0.5mg arimidex can lower serum estradiol levels by almost 100%.
The take away here should be that we cannot infer from studies on naturals how much a given dose of AI will lower estradiol levels in AAS users. The same dosage of AI that will lower E2 by 50% in naturals may well completely tank E2 in AAS users.