The difference being, the level segment at TOC is done at a far higher density altitude than your suggested high-speed cruise climb through the lower altitudes. Higher DA = Lower pressure = lower drag.
Lower altitude = higher air pressure = lower TAS for given IAS = more drag = more fuel used in that climb. Best bet as a rule of thumb? Ignoring wind, get as high as you can, as quickly as you can, and stay there as long as you can are the keys to saving fuel. In normal ops? put your desired CI in the box and let it do the thinking for you. It can do it faster and better than I ever could.