Why do some slave narratives from the WPA seem so positive?

This is a really good question. The interview in question was compiled from a WPA effort during Roosevelt's new deal, The Federal Writers' Project (1935-1939). The super set, "Slave Narrative Collection" was a combination of this federal effort and the follow-up state sponsored effort which came after (1939-1942).

The WPA idea was to send out writers / interviewers paid for by the federal government to both employ writers in worth while make work projects during the Great Depression and to document these important historic stories for future generations. The Slave Narrative Collections were meant to document the views, experiences and opinions of surviving former slavers then 7 decades after the conclusion of the civil war and emancipation before these now very old folks, were all gone.

Question: Why do some slave narratives from EPA seem so positive?

Interview Bias and intimidation. The "federal interviews" were often local white citizens, not journalists, and not unbiased in the life stories they were documenting. Just as the former slaves in question were alive, so were direct relations of former slave owners.

The interviewers were almost universally white folks. Interviewers themselves could be direct relations of former slave holders. These people brought their own bias and intimidation with them to the interviews. Whether conscious intimidation or not. I've had the experience of listening to taped interviews of these meetings and you can hear the interviewers leading and correcting the folks being interviewed. I listened to one interview with a young woman who was very familiar with the man being interviewed, calling him by first name in a familiar tense and referring to her grandfather his old owner. That young woman had clearly know this older man all her life and lived in the same community with him. The Jim Crow segregated south.

It's quite an experience to listen too a young woman tell an older former slave about how much he liked his slave experience. Historians believe that the combination of control over who was interviewed, the bias of the interviewers, and the trepidation of these former slaves sometimes had the effect you are observing in asking this question.

I did hear another tape of a former slave who was not at all intimidated by the interviewer and openly resisted attempts to change his answers, so this explanation is not a universal one.

Here is a list of some of the taped interviews you can listen too from the Library of Congress. So you can hear the interviews yourself.

I think we can also observe that the Federal Government at the time was run by the Democratic Party, as was the Southern State governments throughout this period. These people were in power along with Franklin Delano Roosevelt during the new deal. The Democratic Party was the party at the time of Southern Conservatives who's representatives would be in a position to dictate WPA processes and programs. At the federal level this effort was run for 5 years along with the WPA. When it was defunded in 1939 it would be run by the local state governments in the South in some states for another 3 years.

wikipedia: Slave Narrative Collection.
Though the collection preserved hundreds of life stories that would otherwise have been lost, later historians have agreed that, compiled as it was by primarily white interviewers, it does not represent an entirely unbiased view. The influential historian of slavery John Blassingame has said that the collection can present "a simplistic and distorted view of the plantation" that is too positive.[5] Blassingame's argument proved controversial; one historian in the 1990s described support for Blassingame's position as "rare," but defended him on the grounds that "all historical evidence has to be measured against a minimum standard of truth that would allow historians to use it properly. Historians have not, to date, applied this stipulation to the slave narratives."[6]

More recently, even as the narratives are more widely available through digital means, their use by historians has been more precise, including an examination of responses to conflict among the Gullah community;[7] a history of representations of the black body extending to the present;[8] and a study of the period of their transmission, the 1930s, rather than the period they document.[9] Though most of the narratives are preserved only in the notes of the interviewers, large numbers of photographs and tape recordings were made as well, which have proved valuable for such purposes as examining changes in African American Vernacular English over time.]

Sources:.
United States Library of Congress
wikipedia: Federal Writers Project
wikipedia: Slave Narrative Collection