Virginia Gov. Bob McDonnell (R) has taken heat from education advocates ever since he unveiled a budget plan that withholds inflation increases, diverts some sales tax revenue from schools, and cuts the extra money that Northern Virginia normally gets to attract non-teaching staff in that high-dollar job market.

Gov. Robert F. McDonnell (Tracy A. Woodward/THE WASHINGTON POST)

Why is the governor suddenly opening his checkbook for schools?

Turns out his proposed spending boost springs not from a philosophical change, but a mathematical one.

State numbers crunchers were working with faulty data on K-3 classrooms when they put his first budget together, Daniel Timberlake, director of the Virginia Department of Planning and Budget, told the Senate Finance Committee Wednesday.

Under a program that’s been around since the mid-1990s, the state provides extra funding to poorer schools to reduce class sizes in the earliest grades, Timberlake said. Statewide, the maximum size for K-3 classes is 24 students per teacher. But if a large share of students is eligible for free or reduced-price lunch, money is provided so that class size can drop as low as 14 students.

Budget officials initially worked with free-lunch data from the current budget, Timberlake said. But the number of students that qualify for that program — and by extension, the number of classrooms eligible to shrink — has shot up because of the poor economy, Timberlake told the committee.

“Even though it’s just a revision in data, it does result in lower class sizes,” Timberlake said. “It does achieve a good public education goal.”

The program is discretionary, so McDonnell did not have to add more funding. But it would have been a first, Timberlake said.

“We have always done it,” he said. “Since the program began, there’s never been a time when we didn’t fund it.”