long story short, different companies have different economic circumstances - different assets, different risks, different cashflow, supplier and customer arrangements, different operating structure, different expenses, etc that will all influence the nature of GAAP statements, being a common set of standards applied to ALL companies. It's like measuring all people with a single score like IQ.

Experienced investors never take GAAP at face value - most of them will work with both the GAAP and pro forma (as well as any other public releases) and try to reverse engineer the individual lines in order to tease out more pertinent (in their opinion) factors that affect the value of the business. different investors value risks differently but the pro forma is created by management for a general set of investors in order to bridge the gap between GAAP and exact economic reality.