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We are unable to understand what it is that you are comparing. Seems like you are varying the support conditions of a one span beam and comparing the mid-span moment of those beams with those of a beam in a portal frame where a beam spans between 2 columns like a football goal post.
By LSM, we assume you are referring to the limit state method of design. If so, are you referring to section 4 of IS800-2007? Specifically which clause of that chapter is being used in your comparisons?
Please attach your models and provide specific details of which ones are being compared. Simple diagrams would also make it easier to understand what is being compared.
If your goal is to determine the correctness of the shear forces and bending moments calculated by STAAD for general purpose steel frames, we suggest you go through the verification manual supplied with STAAD.Pro. Several examples are provided comparing STAAD's results with those from widely available textbooks.