Calculation of cylinder flange SW6

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When sw6 calculates a vacuum tower equipment, I choose type B 2.5MPA for the cylinder flange. The calculation result is unqualified for axial stress. When the bolts are adjusted to a smaller size or the number of bolts is reduced, the calculation can pass. I don't quite understand why? Here are the instructions in SW6: This module can design and check calculations for loose flanges, integral flanges, free flanges, reverse flanges and wide flanges. If the verification calculation is performed, regardless of whether the verification is qualified or not, the user will be prompted in the screen output of the calculation results for the minimum qualified thickness of the flange that can meet the design conditions. However, this thickness value is based on the current design conditions and is not necessarily a more reasonable value. Especially when the calculated bending moment of the flange is determined by the pretightening bending moment, reducing the number of bolts or reducing the nominal diameter of the bolts may yield more reasonable results.