It's a general fact that a lot of americans and canadians are ignorant about other nations and what they look like
the most numerous "arabs" in USA are Lebanese and Syrians . so they probably mean that
while some of them are just used to either NW-european americans and some other european americans like italian americans . also afro americans etc. and not west asians therefore they will just think that i dont fit among any of the peoples they are familiar with and so they will think i am "arab"
and yes an exotic greek or portuguese etc. would also be an "arab" for those people most of the time
but as i said there are also nweuro-americans who think i am greek . cant remember if some also think i am jewish too