a 26.0 bar will always be 26.0 in the middle of the bar when the stem clamps down. Everything else from that point can vary. Some handlebars have narrow outside diameters (less than 26.0) but bulge out alot at the stem (to 26.0). In fact almost all drop bars bulge at the stem, otherwise it becomes impossible to mount the bars unless your stem has a removable faceplate.

In response to your question, the levers are designed for 26.0 bars, not a 26.0 ultimate diameter (although it would probably fit anyways). I think the reason most cross levers have hinges is to make up for this variation between bar diameters more than for convenience of mounting.