1. GAAP, which is a man-made set of social conventions for how to present financial results, does not accurately reflect Company X's economic reality and continuing future earnings power, while the pro forma results do. (Or, less strongly, GAAP is informative, but the pro forma numbers are also informative, about a different aspect of the company's situation.) Investors know that it is a useful discipline to hold all companies to the GAAP social convention, but appreciate Company X's efforts to provide them with more insight into its real economic position, and use the non-GAAP numbers to inform their own models.
2. The GAAP numbers are more or less right, the non-GAAP numbers are just fake and flattering, but investors, whose job it is to make intelligent decisions about the future earnings power of the company, ignore the non-GAAP numbers and focus on the GAAP numbers, which are after all reported prominently in the same press release.
3. The non-GAAP numbers are fake and flattering and all the investors are chumps who believe whatever management puts in the big font at the top of the press release.
Pretty much every discussion of non-GAAP accounting focuses exclusively on Possibility No. 3, which, as both an efficient-markets fundamentalist and an accounting post-modernist, I find by far the least plausible of the three possibilities. Anyway, here's a column by Gretchen Morgenson with the pleasingly archetypal title "Fantasy Math Is Helping Companies Spin Losses Into Profits."
http://www.bloombergview.com/articles/2016-04-25/investing-r...