Hortonworks Inc. Chief Executive Rob Bearden forecast in March 2014 that the software firm would have a “strong $100 million run rate” by year-end. But the number looked a lot smaller after Hortonworks went public and then reported financial results: just $46 million in revenue last year.

It turns out that Mr. Bearden wasn’t talking about revenue, though he didn’t say so at the time. The Santa Clara, Calif., company now says the $100 million target was for “billings,” a gauge of future business that isn’t part of generally...