XPMorten,
Pomo1 is right in saying that an increase in speed leads to a lower optimum altitude. The 737 FCM, page 4.6 states "In LRC or pilot selected speed modes, optimum altitude increases as either airplane weight or speed decreases".
The 734 strip that you inserted pertains to LRC control, wherein the effect on increase in alt results in higher LRC speed, and the shaded area is the only one at opt alt. It does not show the effect of increase in speed on opt alt.
If a pilot flies Econ speed, the FMC opt alt reflects min cost, and if a pilot selected speed is flown, then the FMC opt alt is slightly different in that it depicts the alt for min fuel burn.
Having said that, I cannot understand how Pomo1 burnt lesser fuel at a higher Mach No, at a lower alt.