[Qgis-user] Zonal Statistics Computation QGIS2.18.13

[sh_jwa]

I have an issue for the result of QGIS Zonal statistics  mean values. 
I expect the value for the polygon marked red is about 26.6 but it seems
that it is averaged value over one right shifted polygons. 
Values showing in small black font are mean values for the each polygon
according to zonal statistics.
Values showing in blue are raster values.
<http://osgeo-org.1560.x6.nabble.com/file/t381870/zonalstat1.png> 

I made zonal statistics using files with 
tl_2013_us_county.shp   &    us.tmax_nohads_ll_20140525_float.tiff 
on http://www.qgistutorials.com/en/docs/sampling_raster_data.html

1st thing I did was to change CRS for tl_2013_us_county.shp  to EPSG4326
That way, I believe that CRS of us.tmax_nohads_ll_20140525_float.tiff  and
tl_2013_us_county.shp became the same.

2nd I executed zonal statistics 

Platform is QGIS2.18.13 32bit 
On Windows 10 Pro 64bit Intel Core™ i5-6500 CPU

<http://osgeo-org.1560.x6.nabble.com/file/t381870/zonalstat2.png> 

Is there any way to get values what I expect?
For above case, expected value is over 27.2 instead of 26.44.
Could anyone help me please



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