No, I don't think it falls in relation to hcg at all. I had a miscarriage last year after which I suffered from retained tissue. A bit of placental tissue hung on tight inside and kept producing hcg. In the meantime I kept bleeding and passing clots and even went through three or four full cycles with ovulation and af! All, while still with hcg in my blood.
I am not saying your progesterone levels are normally fine as I have no idea how your cycles are but these levels now are no indication of a defect in my opinion.
Since you have already suffered three losses (4 if you count the twin) you can normally expect your doctor to perform a full recurrent miscarriage workup.
This includes checking your hormone levels during your cycle (day 3 and day 21 tests they are usually called although the actual days may vary depending on your cycle), clotting panel, maybe even karyotyping (checking whether your husband or you have a genetic abnormality).
From your side, if you don't do so already, you could try charting and taking your temperature. This could indicate whether you have a luteal phase defect (low progesterone).
You may have to wait six to eight weeks for your hormones to calm down and your cycle to go back to normal for some of those tests.
The good news is that most women who have these tests find a reason for their miscarriages and there is treatment that helps them have their babies. Others, like myself, have to look a little further afield and I am now on treatment for high natural killer cells. This is not a mainstream test and I am guessing your doctor probably won't go for it but it is worth asking about.
In any case, good luck with everything. I hope that they find a straightforward reason and can help you have that sticky bean next time