Raster Cell Size differences between ArcMap and R

Question

I'm having issues when projecting habitat suitability rasters (WGS84) to a projected coordinate system (Africa Albers Equal Area Conic). The reason I reproject the rasters is so that I can calculate the area of suitable habitat which is intended to be used for conservation-related purposes.
Considering that I have about 900 individual rasters that need to be reprojected for area calculation purposes I decided to automate the process in R. However, I realised that there is a difference in 'cell size' values for the same rasters between R and ArcMap 10.7.
For example, when I use 'Raster A' (cell size: X: 0.00833333376786619; Y:0.00833333376786619) and project it to Africa Albers Equal Area Conic the cell sizes are different between R and ArcMap and therefore also the area of suitable habitat...
After reprojecting in ArcMap the cell size of the rasters are X: 879.789038737608
Y: 879.789038737608
But in R the 'cell size' is different: X: 853
Y: 902
The resampling method used was bilinear when reprojecting the raster.
This turned out to be a massive headache to me because I do not know which area calculated is the correct one as both R and ArcMap provide area estimates vastly different from one another. Area calculated from R = 18000km2 and in ArcMap 14000km2.

Mikkel Lydholm Rasmussen · Answer

One of the main difference you see between the two is that R is willing to go for non-square pixels, while Arc enforces exactly square pixels by default.
In your input dataset, your pixels appear to be square, but in fact, in a projected coordinate system, the geographical coordinates are not a square (except at equator). As such, your pixels should not be the same size in X and Y.
However, that does not necessarily mean that R did it best. Without the data, we don't have the full information required to evaluate the problem.
Another approach would be to set the target resolution in the reprojection in R. That way, you could enforce square pixels and thus replicate the ArcGIS results.

Raster Cell Size differences between ArcMap and R

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