Also, remember that any minor equipment changes can make a huge difference in bouyancy/weighting needs. For example: if you were diving in a specific brand of suit in say 3mm, but dove the same thickness in a different suit your bouyancy could be affected. Granted it`s the same thickness, but it does happen.
The same goes for tank types. Say if you used a Luxfer S80 aluminmum and changed to a N80 aluminum, you would need less weight due to the N80 being a bit negative. The same can be said going from brand to brand in a specific model of aluminum or even steel tanks, though the tanks are the same, their bouyancy may be just a bit different.
I`ll give you a better example: I own 2 Luxfer S80 alums and 2 Catalina in the same model. When filled to identical psi, the tanks differ in weight on a digital scale; sometimes by 1/2 pound to 1 pound.