There are a few possibilities...One would be that when you were taking T (shots, patches, gels?) you weren't absorbing it all, since HCG is injected SC you have a better chance of absorbing it. How do your TT levels compare on HCG versus your old protocol?
Other thoughts that come to mind are E2 aromaitzation (which tends to be higher with hcg though), the fact that its more "natural" though I find this unlikely, possibly increased pregnenolone from the testes stimulation improving your overall hormones...couple other things that escape me right now
Bottom line, its awesome you've gotten your shit figured out. When your testes actually function with proper stimulation, your treatment options are more plentiful.