I also saw that extract a couple of years ago and tried to determine who they were talking about. I suspect that the name in the actual will was mistakenly transcribed to be either "Polly" or "Sally" Evans in the 2 different records, the letters can often be mistaken for one another. But either way, I was not able to figure out exactly who they were talking about because there were no EVANS around any of the families mentioned in either 1850 or 1860. It wasn't either daughter of Thomas Evans by those names, because they all lived at home with their family (one was already married in 1844 and was living with her husband & children) in Fort Creek, Granville Co., which was in the southern-most part of the county near the Franklin Co. border. The CHAPMANS (often misspelled as CHATMAN in the census) lived in Nutbush, Granville Co. which was quite a distance away in the northern part of the county. The EVANS family moved to Marysville, Union Co. Ohio shortly before 1860, judging by the ages & birth locations of the grandchildren.
Interestingly enough, there was only one EVANS in Nutbush in any of the census records, and that was Thomas Evans, b. 1825, mulatto,in 1860, living with the family headed by D. STEWART, female age 16; I don't know who he was since that's the only time he appears in the census. With Nutbush being a District that shared a border with Warren Co., which also had a township of the same name, a lot of people living there are often found in the Warren Co. records, and it's possible she may have actually been living in that county. I have researched every EVANS of color in Warren Co., most of them being family members, and I don't know of any Polly/Mary or Sally who would have fit the description of the one mentioned in the will, though.
The other thing to keep in mind, too, is that a lot of Granville folks moved back & forth between Wake, as well as Warren, most of those families were related to one another and it's possible the "Polly" or "Sally" Evans mentioned in the will, only lived there for a short time between the census years.
By the way, there is a map showing the locations of various townships & districts over the years in Granville on the link below.