That was the key question addressed during a finance panel today at Fortune Brainstorm Tech, in the wake of disappointing post-IPO performance for companies like Facebook (FB), Groupon (GRPN) and Zynga (ZNGA).

Egon Durban of private equity firm Silver Lake Partners argued that public market investors used to pay a premium for growth potential, whereas they now have become more focused on the past than on the future. Private market investors, on the other hand, continue paying for what might be rather than what was.

Henry Ellenbogen of T. Rowe Price (TROW) agreed, using the example of EverNote – a provider of cloud-based productivity software that recently raised $70 million in new private funding at a $1 billion valuation from firms like T. Rowe Price:

“If you look at the financials of Evernote when we invested, it’s not worth what we paid,” Ellenbogen explained. “We obviously paid some optionality for what it will be worth in three to five years. But if Evernote quadruples its revenue and tries to raise capital at $4 billion, it wouldn’t make any sense. Earnings may have gone up 4x, but the potential for growth has gone down, not up.”

This valuation gap is likely to be a major topic of conversation later today in Aspen during a panel on tech IPOs, which will include UBS tech banker Mark Zanoli and Zillow CEO Spencer Rascoff.