Cell gets a zenkai boost because he has saiyan DNA in him and Cell can be sensed because he's bio-organic. The numbered androids cannot be sensed because they have no aura. Yeah, it's.. complex. DBZ has like.. three (?) different types of androids in it. You've got the 17/18 type that were human, types like 16 that are all robots, types like Cell that are bio-organic, and  then alien cyborgs/robots. It's just easier to condense it.  :-)