You really can't look at it the way you are suggesting. Setback does not reduce the length of the TT or the reach for a given rider, it is simply a portion of the TTL on the Colnago charts. It is a arbitrary reference point in space that simply reflects the difference in the seat tube angles between the three frame sizes.
Setback = COS(STA)*STL.
Looking at the Colnagos for the sake of argument, the 55STA is 74, The 56STA is 73.3 and the 57STA is 73. The setbacks calculate as 14.6, 15.3 and 16.1 when using the seat tube CTC measurement. Just as it says in the chart.
To get the same fit on a 55 that you'd have on a 57, a rider would have to move the saddle back by almost 1CM, which means that the "virtual TT" would be 55.3 on the 55 and 55.6 on the 57. Virtually identical. The comparison between the 56 and 57 is similar, a .6CM slide back yields 55.6 and 55.6, respectively. Colnago bases their fit on the assumption that a taller rider has a longer femur and that change results in the increase in height. They are assuming that reach (as affected by torso and arm length) is about the same for the riders of those three bikes. They are also assuming that since they build their bikes in 1CM increments that you are going to buy the correct bike for your femur length. If you adhere to their fit philosophy, the 57 will never be "smaller" because a 57 rider would never push their saddle far enough forward to make it fit like a 55. Their leg length would prohibit that.
As I recall, Cervelo does some funky number crunching yielding a value called "reach" which is the TT - Setback.
Last edited by terry b; 11-15-07 at 02:40 PM.