From the ProJo article which Justin cites:

According to the mayor’s chief of administration, Richard I. Kerbel, the retroactive pay for the nonunion officers matches retroactive pay awarded to police union members in arbitration over the last fiscal year

Unlike so many other aspects of public sector compensation, it is, in fact, common practice in the private sector for a company to keep management raises in line with those received by unionized workers. The difference is that companies in the private sector usually do not hand out raises during tough budgetary times. If the Mayor and the City Council had not agreed to raises for rank and file members of the work force, there would be no need for Providence to match the compensation of nonunion officers.
This is in no way intended to single out Providence’s first responders. I am on record as stating that Mayor Cicilline’s contractual “stance” against the firefighters was solely a campaign melodrama (entitled, “Look At Me! I’m Drawing The Line For The Taxpayer!”) that accomplished nothing as it focused on such small portion of the salary column of the city’s budget. Raises across the public sector board in Providence – make that around the state and in the State House – needed to be sharply curbed years ago.