Androgens decrease SHBG levels, whereas estrogens raise SHBG levels. However, some androgens aromatize to estrogens. It turns out that as you increase the androgen levels, aromatase activity is a limiting factor and does not cause a proportionate rise in estrogens, and therefore SHBG levels, so you'll have a higher % of bioavailable test because of two factors: 1) higher androgen:estrogen ratio and 2) lower SHBG:androgen (AAS) ratio.
Also, on a tangent, tren has a particularly low affinity for SHBG.
Here's a good relevant study:
"Peak concentrations of total T increased by 5-fold, but free and non-SHBG-bound T levels were increased by 10-fold after 16 weeks."
Clin Endocrinol Metab 1996 Mar;81(3):896-901
Comparison between testosterone enanthate-induced azoospermia and oligozoospermia in a male contraceptive study. II. Pharmacokinetics and pharmacodynamics of once weekly administration of testosterone enanthate.
Anderson RA, Wu FC.