As has been said, it's mostly a metabolizing thing. I need to take 7 trams to feel like I took one 7.5 hydro. On the other hand, I can take just 100mg of codeine and it hits me like three hydro, because my body metabolizes it into morphine well.
It seems a good sized minority of people get little to nothing from trams. It has to do with how u metabolize certain drugs. I get blasted off trams to this day and I've done H, OCs, hyrdos...