It turns out that growth of charter school students San Diego Unified hasn’t been much of an explosion at all – more like a steady inflation.

The National Alliance for Public Charter Schools, which we’ve cited in stories, miscounted charter school students in a previous report, the group said Wednesday.

The error made it appear as though the number of students who attend district charter schools jumped by about 35 percent in 2012-2013, which would have been the second-largest growth in the nation.

The real number was less than 7 percent. On a list of 50 districts that have the most charter school students, San Diego Unified was actually ranked 18th – behind 34 school districts (some districts were tied in rankings).

Katherine Bathgate, a senior communications manager at the National Alliance for Public Charter Schools, said her group missed about 3,000 San Diego Unified charter school students in its 2011-2012 count.

Bathgate said the mistake happened like this:

In San Diego Unified, there were three schools they thought were “virtual schools” – which they didn’t count toward charter numbers – but were actually charter schools. So really what was wrong was the percentage change from the 2011-2012 school year to the 2012-2013 school year. It was a smaller increase than they thought.

The new numbers add an interesting wrinkle to the school board’s recent claim that San Diego Unified is one of the charter-friendliest in the nation.

School board trustees haven’t said the district has too many charters in general, although it has called areas of town like City Heights “charter-school saturated.”

Trustees John Lee Evans and Richard Barrera have pointed to a rash of failed charters as a reason why the school board should play a greater role in scrutinizing charters’ likelihood of success before they’re approved.

Evans said that if charters fail, it’s the district and taxpayers that pick up the pieces.

Miles Durfee, a regional director for the California Charter Schools Association, said that he questioned the numbers from the National Alliance for Public Charter Schools report when he heard increased chatter from San Diego Unified trustees about charters’ explosive growth in the district.

“What happens is that when we perceive this dramatic growth it gives some people reason to say that we’ve come too far, too fast, and that we need to dial it back,” Durfee said. “The truth is that there’s been more of a steady growth of charters.”