Longitudes and latitudes are not regular in WRF-out files
I am not sure about your R code or your snippet of data.  However, you could do an ncdump on the file and direct the output to text to get it "from the horses mouth" so to speak. 

In general, the lat/long export of the WRF grid will not have a linear pattern.  WRF uses a spatially projected grid, likely an equal area lambert method or something similar. The curvature of the Earth creates non-linear grid values in geographic coordinates, but the projected grid coordinates (which you can also export) are linear. See the example WRF domain below plotted in geographic coordinates.


image credit: Zhongfeng Xu and Zong-Liang Yang