Because there are 2 types of kids applications, one with the assessment of physical presence and one without it as a family application. The physical presence requirement for the second one is waived in favor of "easily" giving the citizenship to children of Canadian citizen, including giving said citizenship at the same time their parents pronounce the oath...
Obviously, if your kid has more than 1095 of physical presence, a separate application can be made, and there's indeed a good chance (but not a 100% chance!) it goes faster than the parent's.