because only an english speaker partaking in casual racism would, imo, label a stereotypical arab as 'achmed'. from what i've seen its an exceptionally rare spelling of the name born of parody of the difficulties english speakers have with the 'h' (which is not the 'phlegm' sound btw)

if we call him tommy he is english, hans and he is german, pierre and he is french... achmed is the arab, with a badly spelled name out of ignorance to boot.