First, your understanding of the ordinary medical use of HCG is really not precisely right. The most common use is to stimulate ovulation in women, not to do anything with regard to suppressed hypothalamic or pituitary function. It has also been used to stimulate spermatogenesis in men, and testosterone production. The same considerations apply.
Second, it isn't the case that optimum dosing for one use and dosing protocol -- e.g., to stimulate ovulation with a shot at the doctor's office, or to stimulate spermatogenesis with again, a doctor's office visit involved for each shot -- is going to be optimum for other purposes, such as stimulating testosterone production with ongoing injections.
Further, it by no means is always the case that commonly used medical dosings are the optimum dosing for most individuals. The purpose could be efficacy in say 95% of individuals, never mind that the dose might be many times that needed for all but the last 10% or some other fraction.
Rather than attempt to reason out why everyone (at least those in the half recommending lower dosages) ought to be wrong on logic and misconceptions such as these, the thing to do would be to look at the evidence.
You have probably already been pointed to a dose-response study in the medical literature, recently published, that validated what I have said on this point. If not, surely it can be found as it has been posted many times recently. Also a Pubmed search would readily turn it up.
So the question should rather be, Why take, on an ongoing basis, a dose that is shown to be pure excess, not delivering higher testosterone than the top end of the amount commonly recommended here?
Especially as HCG use during a cycle is intended merely to prevent testicular atrophy and maintain function. Most certainly one doesn't need to go past the dose that already maximizes testosterone production to do that.
There's also the consideration that you are confused about the entire thing, wanting to use HCG post-cycle. Don't do that: use it as described above. For which the low end of the dosage range commonly recommended here is entirely sufficient.
Its proper use, with regard to steroid cycles, is as a preventative: to have the testes ready to go when LH production resumes.
The wrong use is as you were planning, to fail to engage in such prevention, then stimulate testosterone production via the HCG that will itself be inhibitory to recovering already-inhibited LH production.
I appreciate that someone with a colorful name like Big Cat or something wrote some webpages saying these things, and sure, half of webpages say one thing and half say another. What is needed is judgment as to what makes sense and what has some sound basis behind it and what does not have so much of that behind it.