In the upper gearing, there is a much bigger load on the engine. You will build boost A hell of a lot quicker than in the lower gears. It is not uncommon for my T-25 to hit 16 psi of boost at 2300 in 4th if I am crusing around 2k. (Although my T-25 hits 16 psi by 2800 in second too
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More load = quicker boost = lower flutter limit.