The above may be plausible explanations, but exactly where and when does this information get used? When you have a real problem, you give the authorites the number of persons on board. You do not give a "plus" number, because they don't care. If time permits (and it generally does) you break it down into males, females, children, infants and crew. This is useful information for the RFF crews. Again "plus" is not used. This "plus" rubbish is a left over from the past and has no real purpose today.
Errrr, what? '100 plus 1' is the universally recognised format for passing passenger figures, whether it's to flight deck, cabin crew, ground staff etc. All checkin departure control systems use '100 plus 1' as infants don't have their own boarding card, therefore are counted as a plus one.
I don't buy that either. And whilst RYR may use different weights, infant do weigh something. So by not counting their weight you could be overloaded and/or out of trim.
Infants do weigh something, but they weigh practically nothing. The difference in weight between Adult 1 (a 100kg+ male off on his holidays to Spain) vs Adult 2 (18 year old size 8 female) are far greater than the weight of an infant. Not to mention the fact average bag weights are used.
Ive never heard of a short-haul carrier not doing head counts. Long haul I guess is different.
I only know of one short haul airline that still does head counts, and I work at a very large international airport. The majority of carriers don't even check boarding cards these days, trust is placed with the gate staff. Headcounts generally cause far more issues than they resolve, back in the day it wasn't uncommon to get 10 incorrect headcounts from the cabin crew.