Modal Analysis Comparison with Theoretical Equation (Verification)

Hi everyone!

I am currently doing my final year project on the modal analysis of a truss footbridge and the aim of this study is to verify Staadpro Modal analysis with manual calculation of the frequency. 

It is found that the manual calculation follows the principle from the formula and Staadpro doesnt. 

According to the formula, when the mass is increasing, the natural frequency will decrease and vice versa. While conducting the analysis from Staadpro, it shows that when the mass increases, natural frequency increases too. So my question is what verification can i do when both analysis are not getting the same value