I am following the very simple code found in this question

Converting shapefile to raster: Character attributes

to create a new numeric variable based on a factor. My proble is that the new numeric attribute has a complete different series of numbers than the factor column. For example, instead of reading: 18001, 18002, 18003, it converts these numbers into 786, 894, 895. I do not understand what is going on. I have tried as.numeric and as.integer, and none of these work. For further details, the shapefile column is described as: commune_5m@data$NUMERO: Factor w/ 1842 levels "18001", "18002", ...: 786 894 895

any help would be really appreciated!