But Butkovitz, a frequent critic of Nutter's property-tax effort, said his office had run six different tests, five of which showed the deviation between assessments and sales prices for single-family homes to be much higher - one as high as 35 percent.
He called on the administration to share the methodology used for determining the assessed values and the 13.9 percent deviation.
If the OPA's self-evaluation is incorrect, Butkovitz said, the city could "expose itself to significant financial liability" from homeowner appeals.
Mark McDonald, a spokesman for Nutter, said the administration received the letter late Monday and was still evaluating it.
A difference of 13.9 percent would allow for fairly wide variations in assessments. A house that would sell for $300,000, for example, could be valued anywhere from $258,300 to $341,700 and still fall within the industry standards.
That benchmark also is an average deviation - meaning not every property in the city would be assessed within that range.
Since the results of the reassessment were released last month, a number of politicians, community activists, and property owners have been seizing on seemingly bizarre numbers to cast doubt on the overall assessment.
A recent Inquirer analysis of about 10,000 residential sales from 2012 showed only 40 percent of the homes would be assessed within 15 percent of the sale price.
OPA determined the 13.9 percent benchmark based on five years of sales, with each year weighted to account for fluctuations in the market.
OPA not only removed sales that were not arm's-length transactions between unrelated parties, but also scrubbed sales that were deemed poor indicators of a similar home's worth.
It would be difficult to replicate OPA's findings without knowing the criteria used for screening and removing properties for the self-evaluation - information Butkovitz is seeking.
Butkovitz also is asking for information on how OPA drew up the boundaries for more than 600 "pockets of real estate activity" known as Geographical Mapping Areas.
GMAs also reflect neighborhoods considered to have homes of similar worth, and the boundaries have an impact on individual assessments.
Contact Troy Graham at 215-854-2730 or firstname.lastname@example.org, or follow on Twitter @troyjgraham.