Anastrozol and letrozol are reversible AI. During the use, the organism tend to produces more aromatase to achieve the homeostasis. After stopping the treatment the up-regulated enzimes (sub-physiological levels) and the enzimes reversible inhibited by AI (physiological levels) can lead high concentration (supra-physiological) of aromatase and this can cause estrogenic related side effects.
Exemestane is irreversible / suicide AI. After stopping the treatment only up-regulated (yes, this compound cause up-regulate too) enzimes will be functionaly, and these will be in sub-physiological levels.
The up-regulate of ER will be more linked with the estrogens levels.