The reason why you're not seeing spam on your MacBook is OS X Mail or whatever you're using has a client side (i.e. your computer) spam filter that scans for spam after you download your email.
On iOS, the email is probably remaining on the server and either there is nothing on the server that is filtering out spam or the spam filter is set to a very weak level.
I've never had a Comcast email address so I don't know what they provide you, but I'd guess that you do have some server based spam filtering available. If you have Webmail access, login there and see if you find any options.