This may explain it warwick01
"Normal estradiol Day 3 value -- 25-75 pg/ml
In spite of the above number, generally, estradiol levels about 30 or below -- in conjunction with a high FSH level (high in this case, meaning in the post-menopausal range, i.e. 30-40 or higher) -- is considered menopausal. (Important note: If your FSH hasn't reached post-menopausal levels and your estradiol is on the low side, you are not considered POF or EM. There can be other reasons for low estradiol, including excessive exercise, low body fat, and diminished ovarian reserve.) This is because estradiol levels tend to drop over time. During the first 2 to 5 years following menopause or ovarian failure, blood levels of estradiol drop to an average range of about 25 to 35 pg/ml. Women not on HRT generally will see this number drop even more over time; after about five years, it's common for menopausal women who aren't on HRT to have estradiol levels below 25.
If your estradiol levels are lower than 50 picograms per milliliter, you may still be having a period, but also may be experiencing symptoms of low estrogen -- including hot flashes, vaginal dryness, and sleep difficulties"
Post menopause estrogen levels vary from woman to woman depending on fat levels, diet, health of ovaries, or whether a woman uses Hormones, and maybe genetic factors.